[ramonesfreak]

I have never seen them and probably for good reason. I am against putting predator and pray in the same aquarium. While the crab may stay small and not cause harm (kill fish), it will stress out the other animals if they (fish etc) are small.

Its bad enough to put shrimp in with fish. However, my experience is that if the fish are small enough, the shrimp dont seem the slightest bit bothered

with crab, the fish that are with them might become stressed, might not.

think of this though, you will need to feed those crabs. What will they eat and how will you ensure that it is they that eat it and not the fish? Either they will starve or you will in your frustration, over feed which will cause your water to toxify. In a 12 gal tank, that will happen very quickly

Any animal that lives on the bottom is relying on that food getting to the bottom. if the food is passing through the water column where the fish live......

One must resist the temptation to buy animals just because they are cute or interesting. if you want an ecosystem that is compatible with crabs, set one up

I learned the hard way long ago when my wife wanted a hill stream loach she saw at the store. I warned her they need flowing water over their bodies because thats how they take in oxygen. She begged me, I caved and 2 weeks later it was dead. That was the very last time I made the mistake of introducing animals into an environment that was not compatible.