Pangius, Red Tailed, and Tiger Shovelnosed catfish. Too big for 999 or 1000 fish keepers to handle. Ditto Red Bellied Pacu, and godd*mn Pestco carries them all the time for like $2 each. (And, I am told, they are given to charging the tank wall and breaking it. Silly fish.)
Balloon Mollies and Parrot Cichlids. Quasi-Pisco. Not a big fan of deformed fish.
Discus. Too finicky, too pricey.
Heck, I have a tank of neons and 4 tanks of guppies at the moment. Neons are thriving. I got the guppies for my 6 year old, and kept the spawn around for feeding my turtle and keeping my QT tank cycled.