This is a really peaceful and relaxing concept! I ran into an issue with the viewport size in the browser version of the game where the game did not resize along with it, meaning that the game was tiny in the corner while most of the screen was empty gray space. I also noticed that there doesn't seem to be a limit on breeding fish, so I was able to just keep breeding more and more without buying new ones. Issues with polish aside, I think the concept is great and could definitely be developed further!