They can, but they'll die eventually. They're not meant to be submerged. Expect it to live a few months, and then you'll have to go get another one. Fish generally ignore them, I've heard of Silver Dollars tearing them up, but most people's fish take a taste test and then leave it alone. They're poisonous, so I'd imagine its not good eating. Won't hurt your fish if they do take a chomp though.