My worst purchase was 6 tiger barbs for $30 and gave my entire tank ick killed my clown loach and ate my featherfin synodotis catfish or however you spell it! never again!
My electric catfish... bought it as a cute lil' pudgy thing for $5 a couple years ago. Now it's a big pudgy thing that can't have any tankmates and doesn't do much but sleep, hide and look like a tank bottom decoration. -_-
EBJD = Electric Blue Jack Dempsey. I once got talked into a Chinese Algae Eater, and then got home and learned what terrible aquarium fish they are. While I was waiting for a chance to take it back it became a snack either a Firemouth or a Salvini.