What fish do you have and what tank is it ?
I've kept red claw crabs happily with community fish of various sizes for several years. I normally have them in pairs, although I don't know that, that makes any difference.
A few tips though:
- When you choose the crab, don't buy the dominant aggressive one in the display tank (e.g, the one that's sat on top of the rock, pushing the others off), pick a crab that is hiding under a rock. Much less likely to go for your fish (learnt this one the hard way - of 5 crabs I kept only 1 went for the fish and he was the toughest crab on the block).
- Provide some way that they can get out of the water, they need to be able to get to the air. A piece of bog wood that breaks the surface is ideal.
- Block up all holes to get out of the tank and into your filter! They are very good climbers and will surprise you how they can get out.
- And the first time they shed their shells, don't be alarmed! The shell looks exactly like a dead crab.