This is a difficult question to answer. It depends on the size of the fish and the crayfish. When they get hungry they will eat fish, and it's usually at night when you are not watching. It's happened to me before, so I don't keep them. It gets worse as they get bigger! As to the Gourami specifically, not sure. Mine occupy the upper range of the tank, but crayfish will climb plants looking for food. (yeah I really didn't answer your question)