I have a freshwater red crab?

Well first, if you want another crab the red clawed crab is a great choice. It does eat the plants a bit, but not noticeably, and it doesn't pick on the fish much (it gets a bit territorial if a fish gets too close, but it doesn't hurt the fish). Second you should give it to a friend or something, or you could by a small tank and put him in there by himself. This way he won't be able to har fish. Then just make sure he can't get up into the filter in anyway, by covering it or something. Oh, but no matter what, make sure you have a lid if you choose to put him in a different tank, because he will jump right out if he can get high enough. Same with the red clawed crab.