the crabs would be useful to the puffers, you're right. However they'd still eat the other fish.
Originally posted by ChilDawg
It is most likely that your dwarf would do some damage to crabs, but you have to take into account the fact that crabs might reciprocate. This could cause tetraodotoxins to be released in the tank, at which point you would have to do a large water change (possibly even break down the tank and clean it) and probably say good-bye to the remaining fish/animals in the tank.