One of my fish has ich and i know to cure it you can put copper in your tank and it will kill the disease but i dont know if it is safe for my crabs or if it will kill them.
NO!!! Copper is deadly to invertabrates ( hermit crabs, etc...), there are even copper remover products on the market to remove copper from water that passes through copper pipes in the water system either in your house or to your house. set up a quarantine tank for your sick fish. Look into a copper remover for your tank, it is like the water dechlorinator products.
+1 copper will kill crabs,
