I am adding which fish are based on what in real life, or from what I've seen at least. What the game based its fish on was one of those things that made me curious for a long time, and I could never seem to find any source that showed which in-game fish was based on a real-life fish, outside of maybe one source showing all the "pure breeds" and another showing where Spotanus came from.
Also for notes on the fish I do know so far (using this as a source: http://www.bobbiemelody.com/perfectfish.htm) :
Hornet Arrowfish -- Twig Catfish
Razorback Carp -- Koi Carp
Pink Fatfish -- ???
Orange Fruitfish -- Six-barred Distichodus (Done)
Stubby Grouper -- Black Boxfish (Done)
Finless Quicksilver -- Glass Catfish (Done)
Fanned Snout -- Clown Loach (Done)
Flagged Bananafish -- Fandance Goby
Tiger Catfish -- Plecostomus (Done)
Canary Fire-Arrow -- Guppy (Fancy variation) (Done)
Golden Goldbulb -- Telescope Goldfish (?)
Speckled Leaffish -- Longfin Tetra (?) *Recolored
Silky Shark -- Red Tail Shark (Done)
Twin-Fin Spotanus -- Pajama Cardinalfish (Done)
Greenfin Beta -- Betta (Veiltail) *Recolored
Crimson Comet -- Betta (Halfmoon) (Done)
Oriental Flashfish -- Dottyback (?)
Wasp Goldshark -- Platy fish / Swordtail (Done)
Great-Sail Pigmy -- Sailfin Molly (Done)
Snubbed Snooper -- Guppy (Cobra variation)
Spined Stickfish -- Zebra Danio *Recolored