You really need to have Level 3 environment for healthy fish.

I'm currently making a list of which fish will live as adults (even if they need vitamins) in Level 1. When I'm done with that, I'm going to go on to Level 3.

But my other games have all the research complete and I just need to check in on them for feedings and check for sickness (those games have already found all 7 magic fish.)