1. Clownfish because so many little munchkins just want their OWN Nemo, so their moms, who have not read any thing about the care and keeping of fish, go out and buy a one quart bowl, put in some table salt, and plop the fish in. This makes so many of them go to well-meaning people that just like them but don't know anything about fish.
2. Clown loaches. Self-explanatory.
3. Arapaimas
4. Sharks, period.
5. As much as it saddens me to say it, b-b-buhbahbuhbuh... B-babababa- BETTAS. Many of them go to owners who stick them in a vase with a lily or a "Betta Keeper". I know everyone here on AA and all other aquarium forums know the very least one should be kept in in a one gallon tank, but most non-fish lovers don't, and thus bettas are put in things only a tiny bit bigger than they were in at the pet store (betta breeders are a whole different story, they are good to their bettas, unlike Smetco and Flet Smart.)