Harlie - Harlequin Tuskfish
Egghead - Darkcap Parrotfish (from it’s species name, oviceps)
Pasta Pincher - Brassy Drummer
Loiterer - Two-Spot Red Snapper
Followers - Mosquitoes
Derek - Australian White Ibis
Sneezer - Yellow Wattlebird
Choppers - any quail
Peter Pan - Greencap Parrotfish
Xanthoptilus - a surgeonfish that is either Acanthurus grammoptilus or Acanthurus xanthopterus, two species I still struggle to differentiate
Burger - Spanish Mackerel
Not exactly nicknames, but I shorten the name of many fish species. Often the “fish” part of the name gets dropped, resulting in names like Emperor Angel, Threadfin Butterfly, Pencil Surgeon, Bluespine Unicorn. Sometimes I’ll drop the genus name (so long as this wouldn’t cause confusion), to stuff like orbicularis, clarkii, oviceps, dussumieri, ocellaris, akindynos and percula. Other fish names just get shortened to one word, like Maori, Bumphead, Idol, Spinecheek, Cownose, Blacktip, Whitetip.