Plecos can withstand a lot, which is probably why it's still alive. Did the fish turn a lighter shade when they died? (a sign of chlorine poisoning)

I agree with what's already been said. If the chlorine wasn't removed, that'll kill them really fast. I'm not sure what else would kill them that quickly. Even stress takes a little longer to kill an otherwise healthy fish. Knowing a few more details might help us a little more. I'm sorry all of the fishies died.