Best Cheap Aquarium in 2021
Tetra ColorFusion Starter aquarium Kit 3 Gallons, Half-Moon Shape, With Bubbler And Color-Changing Light Disc
Tetra LED Half Moon aquarium Kit 1.1 Gallons, Ideal For Bettas, Black, 4.6 x 9.1 x 9.9 Inches (29049)
- aquarium KIT Clear plastic half moon-shaped front allows unobstructed viewing
- MOVABLE LIGHT LED light can be positioned to light tank from above or below – requires 3 AA batteries (not included)
- IDEAL FOR BETTAS Bettas have large heavy fins that weigh them down so they do not need a lot of space bettas require minimal maintenance and make a great species for beginners
- CONVENIENT Clear canopy has a hole to make feeding fish easy
- Age Range Description: All Life Stages
- Included Components: Tetra LED Half Moon Betta Aquarium, 4.6 x 9.1 x 9.9 Inches
Aqueon Aquarium Fish Tank
- Ships in own container with reinforced packaging to protect product
- Black trim design with clear silicone seals
- Suitable for freshwater or marine inhabitants
- Aquarium measures 20-1/4" X 10-1/2" X 12-9/16"
- Made in the USA
Tetra Whisper Bio-Bag Disposable Filter Cartridges 12 Count, For aquariums, Large, Unassembled
- UNASSEMBLED REPLACEMENT CARBON FILTER Keeps aquarium water crystal clear – removes odors and discoloration
- CATCHES DEBRIS Dense dual-sided mesh filters debris and fish waste
- FITS TETRA WHISPER WATER FILTERS Color coded to make it simple to remember which replacement cartridge size to purchase
- CONVENIENT Easy to assemble and easy to replace
- USAGE Change monthly or sooner if needed
API FRESHWATER MASTER TEST KIT 800-Test Freshwater Aquarium Water Master Test Kit
- Contains one (1) API FRESHWATER MASTER TEST KIT 800-Test Freshwater Aquarium Water Master Test Kit, including 7 bottles of testing solutions, 1 color card and 4 glass tubes with cap
- Helps monitor water quality and prevent invisible water problems that can be harmful to fish and cause fish loss
- Accurately monitors 5 most vital water parameters levels in freshwater aquariums: pH, high range pH, ammonia, nitrite, nitrate
- Designed for use in freshwater aquariums only
- Use for weekly monitoring and when water or fish problems appear
Tetra 55 Gallon Aquarium Kit with Fish Tank, Fish Net, Fish Food, Filter, Heater and Water Conditioners
- LARGE ENVIRONMENT: Larger environments can house more fish or a greater variety of fish. Maintains water temperature. Essential for tropical fishkeeping
- KIT INCLUDES: one 55 gallon tank, EasyBalance Plus, TetraMin, AquaSafe, 6” fish net, 200W heater, WPF 60 Filter, Stick on digital thermometer, 24” Tetra hinged hood x 2, 2 plant multipacks and a boxwood plant, TetraCare brochure and instruction sheet.
- LED LIGHTING: Included lighting adds the natural daylight effect to your aquarium, giving you illuminated viewing of the entire space.
- WEIGHT: This aquarium tank weighs approximately 79 lb by itself. With water, total tank weight can reach 521 lb.
- COMPLETE YOUR COMBO WITH THE MAJESTY STAND: The Perfecto Majesty Stand (ASIN: B00BUFU77U) properly supports the weight of this tank, while also providing storage for supplies.
25 Foot - Python No Spill Clean and Fill Aquarium Maintenance System
- Will not disturb fish or decor during routine aquarium maintenance.
- Adapts easily to most faucets.
- Complete ready-to-use system.
- No buckets, no siphons, no mess, no tank tear downs ever again.
API AQUARIUM SALT Freshwater Aquarium Salt 65-Ounce Box
- Contains one (1) API AQUARIUM SALT Freshwater Aquarium Salt 65-Ounce Box
- Promotes fish health and disease recovery with increased electrolytes
- Improves respiration for fish in freshwater aquariums
- Made from evaporated sea water for all-natural results
- Use when changing water, when setting up a new freshwater aquarium and when treating fish disease
Tetra Whisper Filter Cartridges 4 Count, Extra Small, For aquarium Filtration (AQ-78052)
- REPLACEMENT CARBON FILTER Keeps aquarium water crystal clear – removes odors and discoloration
- CATCHES DEBRIS Dense dual-sided mesh filters debris and fish waste
- CONVENIENT Replacement filter cartridges are fully assembled and ready to use
- USAGE Change every 2 weeks or sooner if there is reduced water flow
- Age Range Description: All Life Stages
COODIA Internal Green Water Killer Filter Aquarium Tank U-V Light Submersible Disinfection Pump
- Suitable for Saltwater and Freshwater
- Clean up to 150 gallons tank
- Built in 5 watt water pump
- 5 Watt U-V light included
Best Fiddler Crab Tank Mates: What Fish Can Live with My Fiddler Crab?
Fiddler crabs have specific needs and their semi-aggressive attitudes make choosing suitable tank mates difficult. This guide should help the beginner to choose aquarium fish that may live harmoniously with fiddler crabs.
Fiddler crabs are easy to care for and are fairly tolerant and hardy creatures compared to most other aquarium invertebrates. In fact, most fiddler crab owners, even beginners, don't have any problems keeping fiddler crabs whatsoever... until they decide to throw some fish in the tank and "see what happens."
The fact that you have made the effort to research this beforehand is great. It means that you're a responsible pet owner trying to set up a harmonious and interesting aquarium (as opposed to an aquatic fight club or a hobbyist "experiment.")
As a disclaimer, it must be said that the absolute best tank mates for a fiddler crab would be a few decorative accent rocks for him to play with. A single-species (i.e., fiddler crab-ONLY) habitat is the ideal. But we're aquarium owners... we want color; we want interaction; we want life. Let's face it-even a five-gallon aquarium is pretty drab and lifeless with nothing but a lonely crab living there.
The health of your aquaria is important to you, but you should also consider your own mental and emotional wellbeing. You can find yourself in a spiral of depression and loathing if you stare too long at this brownish-grayish animal that may behave like a small lump of clay for many hours each day. Eventually, that fiddler crab aquarium becomes a metaphor for life-the tedium and repetitiveness of it all-with the glass walls representing the hidden but immovable truths that hold us all back. Next thing you know, you have to go on lithium to stop you from crawling around your apartment eating garbage and dead insects that you pinch out of corners with a hand that you refer to as "the little claw".
Add a few pretty fish in with the fiddler crab and you can avoid this dark chapter in your life.
The assumption is that you're planning to either 1.) add several fish to a one- or two-crab habitat, or 2.) planning to add a single crab to an established tropical fish aquarium. If your intention is to add one or two fish to a colony of a dozen fiddler crabs, there are no "good" tank mates. (If you're dead-set on it, do your crabs a favor and pick a fish that is fatty and/or nutrient-rich...) For a "normal" home aquarium setup, the fiddler crab is going to be the anomaly. And any anomaly-especially an anomaly that waves a big-ass claw around-warrants special consideration. In short, we need to consider the fiddler crab itself before we consider his tank mates.
A pretty consistent fact about all fiddler crabs is that they will eat anything. This is one reason they make cool pets. It can be hilarious to watch them use their mismatched claws to break down "human food" to bite-sized pieces. In addition to a staple of sinking shrimp pellets, fiddler crabs will happily take romaine lettuce, molding wheat bread, cooked frozen (human consumption) shrimp, and leftover baby food. They even help you keep the tank clean by eating algae and other semi-edible scum.
Oh, and fiddler crabs love to eat fish.
"Fiddler crabs will eat anything" means "FIDDLER CRABS WILL EAT ANYTHING". This includes living anythings. If it is close enough, small enough and slow enough, a fiddler crab will grab it. And if a fiddler crab grabs it, a fiddler crab will eat it. That's obviously the number one consideration when choosing tank mates for a fiddler crab. Slow fish are a problem. Overly-brave fish are asking for trouble. Long fins are a hazard. Bottom-dwellers need to watch their backs.
If you already have a crab habitat and want to add new fish, it's important to decide exactly how many fish (or how many "inches of fish") you're going add before you begin choosing the species themselves. It's easy for aquarium owners to experience "kid in a candy store" syndrome when they arrive at their local fish retailer; instances of beginners overcrowding their aquariums is unfortunately the rule rather than the exception.
Overcrowding of fish becomes an even greater concern when there are crabs waiting for them. The higher the ratio of fish per gallon, the higher the chance that one of them will end up within reach of a crab. Even if all of the fish are faster than the claw, overcrowding eventually leads to death. If a dead fish does not float to the top or stick to the filter intake, your fiddler crab will gladly perform his own special service for the departed. Fiddler crabs should not actively "hunt" any fish in a healthy aquarium. But if a fiddler crab is eating dead fish regularly, logic follows that he will begin to see fish as a primary food source.
Incidentally, overcrowding of crabs actually makes them more docile. Fiddler crabs act most naturally in small groups. "Natural" behavior is not necessarily the ideal, as it means that they will territorialize the habitat and fight over those territories. (They're naturally crabby, after all.) Crowding fiddler crabs into a habitat up to a point where territorialism becomes impossible is an effective means of sedating the crabs. This is used to good effect by dealers, but does not create a happy place for fish... you would be carpeting their home with things that want to eat them. One fiddler crab can live comfortably in a five gallon tank. Two fiddler crabs should be considered maximum capacity for ten gallons; moving into the 20-30 gallon range will make it possible to support a population of three, or make territorialism more comfortable for two.
This all brings up the question of bioload, or "how many animals can I house in X gallons of water?" There is a unique problem here because the generic formula for freshwater fish is "1 inch of fish per one gallon of water" and the standard for fiddler crabs is "one fiddler crab per one square foot" (which refers to the surface area of your aquarium's floor.) I tried to convert this but came up with a ratio of three inches of fiddler crabs per 12 fish of gallons expressed in light years squared, which seems incorrect.
My personal solution is to simply slash the available bioload for fish in half when combining with any fiddler crabs, whether you have one or five (basing the crab population on a loose adherence to the "one fiddler crab per square foot" guideline.) So a ten gallon aquarium can house one or two crabs with up to five inches of fish. Or if you have a five gallon habitat, you can put two small fish in with your fiddler crab. This formula is not based on any specific science. It's just an effort to err on the side of caution and a safeguard against the fact that I know you, as an aquarium owner, are going to bend any rules given to you anyway. (I know this because I keep 10.5 inches of fish in a ten gallon aquarium.)
If you're looking at adding a new fiddler crab to an established aquarium, there are additional benefits and risks. The idea is attractive because it minimizes the stress on the fish involved (versus the stress induced by driving a new fish home from the retailer and dumping him in an unfamiliar environment with a monster crustacean.) Also, fiddler crabs can tolerate a pretty wide range of living situations. Acceptable water temperatures for fiddler crabs range from 65 to 85 degrees Fahrenheit-this makes them compatible with basically any heated or unheated home setup. (To maximize health and comfort, the best water temperature for fiddler crabs is between 75 and 82, which overlaps with the recommended temperatures for many popular aquarium species.)
Although a heated debate rages on, it's generally accepted that fiddler crabs can live in either of two completely different types of aquariums: freshwater or brackish (somewhat salty.) Freshwater is the norm for most retailers and hobbyists despite many "experts" on the internet sitting on the brackish side of the fence. But the loss of a few trace minerals (whose benefits to fiddler crabs are vaguely defined in the first place) is generally accepted when considering the alternative of trying to mix, measure and maintain legitimately brackish water. In addition to the knowledge, equipment and time required to manage a brackish habitat, the low availability and high cost of brackish species make it an unattractive option for most hobbyists. I've personally found that the use of "aquarium salt" (a low-cost additive for freshwater aquariums which does NOT create a true brackish environment) along with rotating vitamin-rich fruits and vegetables into their diet will keep fiddler crabs active and healthy in freshwater environments.
Which brings us to the first downside of adding a crab to an established aquarium... you really need to put "non-fish" foods into your fish tank to adequately vary your crab's food sources. And they're going to turn your aquarium gross if the crab doesn't eat them in their entirety right away. This effect can be reduced by placing crab-specific food on an item that extends from the aquarium floor to above the surface so the crab can crawl up and eat. If this sounds like too much of a hassle, you're not going to like what comes next.
Fiddler crabs need access to air. Retailers may tell you otherwise, based on the fact that all their crabs live a "long time" in a generic freshwater holding tank. It's true that they may live for months fully submerged, but they will eventually die as a direct result of this living situation. Arguments to the contrary are typically based on the passing of misinformation from "experts" (who are generally salespeople trying to move product.) Your fiddler crab will need to have a rock, ornament, or piece of driftwood that extends above the water's surface. (Creating a "beach" by piling sand or gravel at one end of the tank is a typical solution in crab-only habitats, but makes cohabitation with fish difficult, as it naturally calls for low water levels.) A fiddler crab will spend a significant amount of time with at least some part of its body out of water. This may require significant re-thinking and rearrangement of your established fish tank as you must not have the air access object arranged in a way that would allow your fiddler crab to escape the tank.
Fiddler crabs also like to escape aquariums via airline tubes and filter snorkels if given the chance. These items are essential to a healthy environment for fish and crabs alike; running an unfiltered aquarium is not a good solution. Overcoming these challenging circumstances may require a combination of a tight-fitting aquarium cover, shortened filter snorkel, and meticulous habitat arrangement. An escaped fiddler crab is generally not ideal for the owner. Even if the crab avoids being misidentified as a "disgusting huge spider-scorpion thing" and subsequently stomped on by your wife/mom/child, he will die a less dramatic death within a few hours of being removed from water.
Now that you know all of the things that are going to go terribly, disastrously wrong when you mix fiddler crabs and aquarium fish, let's get down to specifics and look at some potential fiddler crab tank mates!
Fiddler Crabs with Goldfish?
This is not advisable for the same reasons you shouldn't mix goldfish with tropical fish. Goldfish are susceptible to disease and good at spreading them to other, less hardy creatures. They are also dirty fish in terms of waste output, which will inevitably result in your crab trying out a nice hot clawful of diseased goldfish crap as a potential food source. Of course, the fish would have to be alive long enough to produce the aforementioned crap, which probably won't happen due to their nonchalant swimming style and big, fan-like fins.
Verdict: goldfish and fiddler crabs don't mix.
Fiddler Crabs with Zebra Danios? (Fiddler Crabs with Glofish?)
Zebra Danios may be the best possible tank mates for a fiddler crab (short of another fiddler crab.) They're small, peaceful, and a lot faster than a fiddler crab. They tend to swim around the upper two-thirds of an aquarium, safely above the substrate where a fiddler crab spends most of its time. Danios live in schools and should follow one another's lead in avoiding a crab attack. But schooling also means that bioload can be an issue even in spite of the tiny size of each individual fish. For the comfort and health of the fish, you should have a school of no less than five.
Totally rad strains of neon-colored Zebra Danios are marketed under the name "Glofish". The intense colors are apparently coaxed from a naturally-occurring gene. Glofish are perfect for adding color to a boring crab habitat.
Verdict: Zebra Danios and fiddler crabs can live together, pending available space for schooling.
Fiddler Crabs with Tetras?
There are a ton of different tetra varieties out there. All of them should be pretty safe bets with a fiddler crab since every tetra I know of is short-finned and extremely quick. Much of what was said for Zebra Danios applies to tetras as well.
But tetras will be slightly more adventurous than Zebra Danios on average and may explore the option of nipping at fiddler crabs. If anything, this will just make your tank more interesting-fiddler crabs are wary of every movement (even outside of the tank) and will often make a preemptive swipe when the tetra is still in exploration mode, before it's even within nipping range. At this point, the tetra's reaction time will be more than sufficient to dodge the swipe but it will abandon any ideas of tracking and tasting the fiddler crab... for a while. If the tetra does manage to strike first, a nip to the exoskeleton is very unlikely to harm your fiddler crab in any way.
Verdict: tetras and fiddler crabs can live together, pending available space for schooling.
Fiddler Crabs with Barbs? (Fiddler Crabs with Tiger Barbs? Cherry Barbs?)
Out of the various tank mates I have personally tried for fiddler crabs, Tiger Barbs were probably the most fun to watch... until they died.
The whole ineffective nip/swipe routine that can occur between tetras and fiddler crabs is amplified with barbs (listed as semi-aggressive by many retailers). I had three Tiger Barbs and it often appeared as if they were schooling around the fiddler crab instead of each other. The crab didn't seem harmed or distressed by the attention and the fish were obviously not living in fear.
But then all three tiger barbs died within a few days of one another. Barbs can be sensitive to fluctuating water parameters and I suspect that sharing a limited living space with a fiddler crab was just not healthy for them (ten gallon aquarium reduced to 2/3 water level for the crab's air access points). One of them might still be hiding in there... that particular investigation never yielded a body. Incidentally, the crab did not come out searching for food that night.
Verdict: barbs and fiddler crabs can live together, pending a large habitat and increased attention to water quality.
Fiddler Crabs with Livebearers? (Fiddler Crabs with Mollies? Guppies? Platies? Swordtails? Lyretails?)
All of these livebearer species are supposed to be peaceful "community" fish. My own experience is that this class of fish has consistently inconsistent behaviors. This is backed up by reports of aggressive or random behavior by countless other aquarium owners. Behaviors in these fish can change drastically if the ratio of male to female livebearers in the tank is changed, or due to mating/pregnancy issues (which are constant with these naughty, naughty fish.) Aggressive tendencies may be manifested only within the livebearer population, or against other species in the aquarium as well.
Guppies, platies, mollies-they're all weirdofish if you ask me. Case in point: two female platies who took up residence in my fiddler crab's domain after the tiger barbs vacated. Everything was great for about a month. The platies usually stuck to whichever end of the tank the crab wasn't at, and easily dodged his infrequent swipes at them.
Over time, it appeared that one of the platies was getting fatter. She must have gotten pregnant before coming home with me (the gold digging skank.) I removed her from the aquarium to put her in a temporary birthing tank. At that point I noticed she had a ripped fin and was swimming kind of lopsided; she also had a dark red spot on one side of her body. Her condition declined quickly and she was dead within an hour.
When I went back to the aquarium to check on the other platy, she was hiding under a rock with just the tip of her head peeking out. I hadn't looked there because neither of these platies had ever hidden before that day-they were both very much fans of open water. Stranger yet, this was the exact same hiding spot the fiddler crab frequently used. This very platy had been swiped at by a fiddler crab hiding in this very same location several times before, so it seemed like a fairly bad hiding place... the complete opposite of a hiding place, really. I now firmly believe that fish do have feelings-I had a suicidal platy.
I eventually got the depressed platy to come out of the crab hole and I moved her to my "regular" fish tank, where she continued to hide for three days. She had no physical injuries that I could see. I'm happy to report that she is once again behaving like a normal fish... for now. To this day I don't know if there was a platy fight or if the crab attack.
Verdict: fiddler crabs can live with regular platies or mollies until they start acting weird. Guppies, swordtails, hi-fins, and lyretails provide significantly larger fin surface area which could be the focal point of a crab attack, and would therefore be at risk.
Fiddler Crabs with Bettas? (Fiddler Crabs with "Siamese Fighting Fish"? Beta Fish?)
Using a betta as a fiddler crab tank mate might be an even worse idea than using goldfish. The betta will not be scared of the crab. If the betta feels irritated or defensive, it will strike at the crab. There are two big differences between this and the "cat and mouse" game that fiddler crabs will play with tetras and barbs. First, a strike from a healthy betta has the potential to actually harm a crab if it attacks an eye or a joint on one of those delicious little legs. Second, the betta is not likely to escape the encounter. It isn't a particularly fast fish and it has fins (or "crab handles", as I like to call them) trailing behind it for miles. Bettas also tend to just kind of float around when there are no stimuli present, and they use the full vertical range of their tank, including the bottom. Those both contribute to the potential for a betta to get crab-grabbed even if it accepts the crab's presence and doesn't initiate the encounter.
Verdict: fiddler crabs and betas don't mix.
Fiddler Crabs with Gouramis? (Fiddler Crabs with Dwarf Gouramis? Kissing Gouramis? Gold Gouramis? Labyrinth Fish?)
Gouramis are closely related to bettas-they're both categorized as "labyrinth fish." The family resemblance is clear in the lazy, floating behavior shared by gouramis and bettas. Also like bettas, gouramis travel the full vertical range of their environment, regularly visiting the floor. Popular aquarium species like the Dwarf Gourami can be easily spooked by external sounds or movements, and will check into the nearest cave or hole at maximum velocity. Incidentally, fiddler crabs also love to hide in caves.
The biggest difference between a gourami and a beta is that a typical gourami is peaceful to a fault and will consider death to be a better option than counterattacking against a fiddler crab (or any other aggressor).
The scenario goes something like this... you get into a fight with your girlfriend on the phone and scream, "FINE THEN, YOU'RE OFF MY TOP FRIENDS FOR THE REST OF THE WEEK!" Your poor dwarf gourami gets freaked out and heads for the open mouth of that kick-ass skull ornament you have in your tank. Your fiddler crab happens to be hanging out in a concealed location scraping algae off the skull roof. The crab abandons the algae for a much more fulfilling meal. And you have the worst day ever when you notice your big, friendly gourami twitching out his final, futile attempts to escape the grip of the crab's big claw. Nice going, you jerkwater gourami killer. Even if the gourami is fast enough to escape the crab attack, they're sensitive to stress and can eventually die from dealing with a surprise crab attack every time they try to check into a cave.
Verdict: fiddler crabs and gouramis don't mix.
Fiddler Crabs with Bottom Feeders? (Fiddler Crabs with Plecos? Corys? Algae Eaters?)
Problem number one is that none of these "cleanup crew" fish are particularly fast or agile. Ever see a cory try to swim to the top of an aquarium? It looks like a fat kid trying to get out of a pair of tight pants.
Fish whose main preoccupation is eating scum are going to have a natural problem getting along with fiddler crabs in the confines of a home aquarium. Bottom feeders and invertebrates both spend the majority of their time on the tank floor, and largely compete for the same food source. This competition will stop when your bottom feeders become part of the food source.
Verdict: fiddler crabs and bottom feeders don't mix.
Fiddler Crabs with Cichlids?
The ideal for cichlid keepers seems to be an "active" community rather than a single fish. People like these fish not in spite of the fact that they're aggressive, but because they're aggressive. Even if your cichlid swarm is unable to kill your fiddler crab, they'll eventually stress him to death. Plus, cichlids aren't cheap so you probably don't want their fins ripped off during the constant conflict.
Verdict: cichlids and fiddler crabs don't fix.
Fiddler Crabs with Puffer Fish?(Fiddler Crabs with Green Spotted Puffers?)
This may seem like a great idea if you're setting up a brackish environment. The Green Spotted Puffer is vibrant, easy to find, and relatively cheap compared to a lot of other brackish aquarium species. But that cute 1½ inch puffer you got at a department without doing any research is going to get a lot bigger. Then what happens?
Answer: look up "fiddler crab vs. green spotted puffer" on YouTube. There are several informative videos available...
Verdict: fiddler crabs and puffer fish don't mix.
Fiddler Crabs with Other Invertebrates? (Fiddler Crabs with Shrimp? Fiddler Crabs with Crayfish?)
A common report from those that have tried cohabitating shrimp and fiddler crabs is that everything seems fine at first with the different species staying to themselves. A day or two later some shrimp are mysteriously missing legs. (Fiddler crabs are great at disposing of evidence.)
Until proven otherwise, it should be assumed that a fiddler crab will be unconditionally unfriendly towards any living thing that happens to be in their "personal space"; this can be manifested as either aggression or fear depending on the size of the intruder. Tank mates must be chosen accordingly. This is easy to forget because fiddler crabs are simply too slow to pose a real threat to most aquarium fish. Their attacks appear benign, even comical. A fiddler crab making a grab for a tetra looks more like an attempt to shoo the tetra away than an attempt to actually lock the fish in its bone-hard claw and slowly eat it alive, tiny piece by tiny piece.
But a bamboo shrimp can't dart off into open water to avoid a claw the way a tetra or barb can. Just because an animal is a fellow invertebrate doesn't put it in a special don't-eat-me brotherhood with your fiddler crab. In fact, it usually just means they're more susceptible to a crab encounters because they inhabit the same spaces. I mean really, isn't a shrimp pretty much just a little crab with no means of defending itself? There's no reason for a crab to not snap off a shrimp leg for lunch every now and again.
Incidents of fiddler crabs and crayfish fighting each other should be limited by the similarity in size and disposition. They're both known to eat whatever is within grabbing distance, but neither is known to go out of their way for the sole purpose of picking a fight with an animal that's too big to eat-or one that might try to eat them back.
Fiddler Crabs with Live Plants?
Fiddler crabs will uproot, knock over, or try to eat just about any plant commonly used in an aquarium. If you can arrange your tank to avoid this behavior, it's always nice to use live plants in any habitat.
Verdict: live plants can live with fiddler crabs, but can be difficult to manage.
Having gone through all of that, I must note that any and all species-specific information given in this article is subject to be completely and totally wrong at any given time. Each individual fiddler crab and each individual fish are unique animals. Unfortunately, advice about tank mates in any situation can only be given based on generalizations, past experience, and educated guesswork.
You may find a fiddler crab with no interest in eating anything that moves. In this case, you might slowly introduce riskier tank mates and find that the aquarium exists in total harmony until the animals die of old age. On the other hand, you may find that the antics of a particularly curious new zebra danio put this docile crab over the edge and induce a tank-wide bloodbath. Nobody can say with 100% accuracy how any two animals are going to react to one another's presence until it they are actually put together.
I don't need to get a bunch of replies to this article letting me know how you've had a fiddler crab and a gourami living together in a 1.5 gallon tank with no problems, or how your parents who have been happily married for forty years are a betta and a fiddler crab. For one, I don't care. Worse than that, your rare success could inspire a beginner to try a potentially devastating pairing of aquaria. I'm not saying you're a liar; I'm saying you're lucky-it happens. Just like two "peaceful" guppies going nuts and battling to the death also happens.
Because of their size and availability, it's hard to remember that fiddler crabs and aquarium fish are all essentially wild animals, even if the strains have been bred in captivity for a few generations. Each animal is an individual, with an individual personality. Every member of your aquarium community may have something that makes them "tick."
The best rule to remember is that no rule can ever apply 100% of the time when you mix fiddler crabs with other animals, or even with each other, in captivity.