The most generic fish monster is the CR 0 Quipper (MM 335) which is also referenced in Find Familiar, there is no more generic fish monster than this option.
A quipper is a carnivorous fish with sharp teeth. Quippers can adapt
to any aquatic environment, including cold subterranean lakes. They
frequently gather in swarms; the statistics for a swarm of quippers
appear later in this appendix.
Find Familiar (PHB 240):
You gain the service of a familiar, a [..] fish (quipper)
You can find the Swarm of Quippers in the MM on page 338 (for swarms in general for example of generic critters like rats, insects, spider etc see also The Nature of Swarms MM 337)
As the DM, you may call a fish by any name, if you want to point out that the fish are any specific fish like bass, trout or carp and if needed use the statistics of another fish (read Modifying Creatures, MM 6).
For less generic fish monster options (only beasts, snakes excluded), see the CR 0 Octopus (BR 146), the CR 0 Sea Horse (BR 152), the CR 1/8 Dolphin (VGM 208), the CR 1/2 Giant Sea Horse (MM 328), the CR 1/2 Reef Shark (MM 336), the CR 1 Giant Octopus (MM 326), the CR 2 Plesiosaurus (MM 80), the CR 2 Hunter Shark (MM 330), the CR 3 Killer Whale (MM 331), the CR 5 Giant Shark (MM 328).