breeding?

[Le Zeplin]

Do the fish reproduce or is this not an aspect in the game

[SilvermistInc]

Nope

[a34573]

It is only an aspect in the DLC, Freshwater Frenzy, where many of the freshwater fish reproduce.

For the main game and the fish introduced in Architect's Collection, the fish do not reproduce.

[fubberpish]

In the base game, there is no breeding. The Freshwater Frenzy DLC includes breedable fish, and some mods contain breedable fish (but breedable fish in mods require Freshwater Frenzy to function).