they do dissapear after awhile but they must be properly acclimated as they probably died.
best is to drip them.
i won't suggest this in a big tank because they are slow workers and you need many. and not many people will borhter dripping them, and moreover, they will get sucked into wavemakers.
regarding poisoning, if carbon is runned and tank is big like jack's tank, no problem. in fact these guys won't crash the tank when die.