Fiddler crabs may be kept with several other animals, but the tank should be catered to their needs, and not vice versa. Brackish fish such as mollies, guppies may be kept. Dwarf puffers have been known to eat crustaceans. While I have not kept them with fiddler crabs, it may be a risk.
Fiddler crabs should get along with other fiddler species, but try this at your own risk, as no one has tested compatibility with all the fiddler species. I currently keep U. minax and U. pugilator together with no problems. U. minax, U. pugilator, and U. pugnax can be found coexisting in the wild.
Red clawed crabs – Fiddler crabs can coexist with red clawed crabs, which have similar care requirements. Keep in mind that red clawed crabs’ claws are much more powerful, and that they tend to require more water than land. In my experience, larger RCCs can and will break off legs and claws on weaker fiddler crabs. For this reason, I was forced to remove RCCs from my tank. Mix with caution.
Mudskippers – Larger mudskipper species, such as P. barbarus, are known to eat crabs in the wild. I had attempted keeping a medium sized species, P. septemradiatus. However, while the mudskippers could not fully consume the fiddlers, they did nip of legs. The crabs also became extremely stressed, and deaths resulted. For this reason, I would warn against keeping mudskippers and fiddler crabs together. The smaller and more docile indian mudskippers might be compatible, and I will be testing this sortly.
A natural daylight cycle will benefit fiddler crabs as it will all animals. Lights on an 8-12hr timer should work well. Strong lights are not required, any fluorescent or even a shop light will work. It is thought that hermit crabs benefit from full spectrum lighting including ultraviolet spectrums(UVA and UVB) and fiddler crabs may benefit from this as well. This can be provided by a reptile light bulb such as ReptiSun. However, from my experience, it does not seem necessary, and I have not provided UV exposure in my enclosures.
Mold can become a problem in crab tanks. A lot of water tends to evaporate and in a sealed tank, causes high humidity. Humidity is good for crabs, but may also cause mold to grow, especially on wood. Mold is most likely not very harmful for your crabs, but is not very aesthetic either. Leaving the hood partially open or aerating with an air pump or fan may help.