Erp, way to go, aquarium dumpers
Seriously, if the pet you released into the wild isn’t going to be dinner for the local wildlife, it will upset the ecosystem (and by that, I mean eat the prey that was meant for other predators, slowly wiping them out, etc).
(Heck, there’s a lot of controversy over letting domestic cats run free range outside.)
(In somewhat related news, I was reading a message board thread on betta euthanasia techniques, and flushing a live fish down the toilet is considered an unacceptable method of euthanasia, since the fish doesn’t actually die right away, and is laying in human waste, waiting for death. And heck, there’s plenty of water down there to keep them alive, despite the human waste part.)