Originally Posted by crister13
Don't flush them thats horrible. They look kinda like porcelain crabs. I would keep them. They won't kill anything at that size.
Without a definitive ID on the crabs it's impossible to know if they're capable of killing anything now or later on.
People inject aptasia to kill them all the time. They feed ghost shrimp to large fish, brine shrimp to small ones, they toss unwanted snails from planted tanks, and poison or trap pests in their own homes. When a pest arises it's an aquarists obligation to remove it from the aquarium in the same way that we remove fleas from the family dog or cat. There's no moral issue to be debated here.
Now, if it can be positively identified as a completely harmless crab that won't attack the other animals the person here intends to keep, then that's another matter. At best, I'd ask a LFS
if it wants it, otherwise it's best to send it packing. I can't imagine a fish shop would take it in. It's a risk that most shops would never consider and wisely so.