Blue crabs will survive just fine in fresh water. Simply acclimate them slowly to fresh water. They are eating machines, Will eat creyfish, Snails, insects or any thing can pin down. They would eat dead bullheads for sure. they do get large in fresh water so if you put a bunch of small ones you caould have very large one. Also they will eat each other. you could try it but if you put a bunch they might eat every thing first then start kiling each other...
