I have a 30 gallon tank (filled 2/3rds of the way up) with 2 filters, and usually 2-3 FBT's. The FBT basically live on the islands on top. and the crabs typically live on the bottom, except for when they come up for air. So I mean, it's a big enough tank, that whatever poison the toad's excrete, it's dilluted enough that it doesn't bother the crabs. I've had crabs live for over a year in the tank.