Well, something is just not right in your games. You should never get Leaffish from common eggs, so that's not right. The only fish that should hatch from common eggs are Sharks, Betas, Fatfish, Fruitfish, Spotanus, and Stickfish - and only certain fin types of those species. I wonder if something got corrupted in your game, somehow, because you're seeing things that just shouldn't happen. I wonder if, somehow, the Unknown Chemical changed your game in an unexpected way? You haven't had any game crashes, have you?

One thing that might provide a clue to what is happening is to look at the Species Chart for the fish that are dying and compare it to the ones that are doing OK. If the fish that came from eggs don't show the same parents on the Species Chart, something is wrong. I'd be especially interested in what's on the Species Chart for the Leaffish and any fish that are dying for no reason.

I can envision a couple of options about where to go from here. One is to keep going with the fish that you can get to survive until you have Level 3 of Fish Environment, and see if that allows you to keep the fishies alive. The other is (sorry to say) to delete all of your savegames and/or uninstall the game and reinstall it to see if that corrects the game's behavior. We have not seen that happen in testing.