The real-life species equivalent of every fish from Animal Crossing: New Horizons. With the arowana some guesswork was necessary, I added my thoughts on the logic next to it.

This was made possible thanks to those editing Nookipedia, and this list:
https://nookipedia.com/wiki/Fish/New_Horizons

Bitterling - Acheilognathus melanogaster
Pale chub - Zacco platypus
Crucian carp - Carassius langsdorfii
Dace - Tribolodon hakonensis
Carp - Cyprinus carpio
Koi - Cyprinus rubrofuscus
Goldfish - Carassius auratus
Pop-eyed goldfish - Carassius auratus
Ranchu goldfish - Carassius auratus
Killifish - Oryzias latipes
Crawfish - Procambarus clarkii
Soft-shelled turtle - Pelodiscus maackii
Snapping turtle - Chelydra serpentina
Tadpole - Hyla japonica
Frog - Hyla japonica
Freshwater goby - Odontobutis obscura
Loach - Misgurnus anguillicaudatus
Catfish - Silurus asotus
Giant snakehead - Channa micropeltes
Bluegill - Lepomis macrochirus
Yellow perch - Perca flavescens
Black bass - Micropterus salmoides
Tilapia - Oreochromis niloticus
Pike - Esox lucius
Pond smelt - Hypomesus nipponensis
Sweetfish - Plecoglossus altivelis
Cherry salmon - Oncorhynchus masou
Char - Salvelinus leucomaenis
Golden trout - Oncorhynchus aguabonita
Stringfish - Parahucho perryi
Salmon - Oncorhynchus keta
King salmon - Oncorhynchus tshawytscha
Mitten crab - Eriocheir sinensis
Guppy - Poecilia reticulata
Nibble fish - Garra rufa
Angelfish - Pterophyllum altum
Betta - Betta splendens
Neon tetra - Paracheirodon innesi
Rainbowfish - Rhadinocentrus ornatus
Piranha - Pygocentrus nattereri
Arowana - Scleropages formosus (Nookipedia lists only Osteoglossinae, though the closest match is likely the Asian arowana Scleropages formosus)
Dorado - Salminus brasiliensis
Gar - Atractosteus spatula
Arapaima - Arapaima gigas
Saddled bichir - Polypterus endlicheri
Sturgeon - Acipenser medirostris
Sea butterfly - Clione limacina
Sea horse - Hippocampus coronatus
Clown fish - Amphiprion ocellaris
Surgeonfish - Paracanthurus hepatus
Butterfly fish - Chaetodon auripes
Napoleonfish - Cheilinus undulatus
Zebra turkeyfish - Pterois lunulata
Blowfish - Takifugu rubripes
Puffer fish - Diodon holocanthus
Anchovy - Engraulis japonicus
Horse mackerel - Trachurus japonicus
Barred knifejaw - Oplegnathus fasciatus
Sea bass - Lateolabrax japonicus
Red snapper - Pagrus major
Dab - Limanda limanda
Olive flounder - Paralichthys olivaceus
Squid - Loliolus japonica
Moray eel - Gymnothorax kidako
Ribbon eel - Rhinomuraena quaesita
Tuna - Thunnus orientalis
Blue marlin - Makaira nigricans
Giant trevally - Caranx ignobilis
Mahi-mahi - Coryphaena hippurus
Ocean sunfish - Mola mola
Ray - Hemitrygon akajei
Saw shark - Pristiophorus japonicus
Hammerhead shark - Sphyrna lewini
Great white shark - Carcharodon carcharias
Whale shark - Rhincodon typus
Suckerfish - Echeneis naucrates
Football fish - Himantolophus groenlandicus
Oarfish - Regalecus glesne
Barreleye - Macropinna microstoma
Coelacanth - Latimeria chalumnae