With Level 3 of Fish Environment, your fish shouldn't be losing health unless they're sick or there's a problem with feeding frequency. Have you just purchased Level 3, or has the research completed? If it doesn't say "This is fully researched" the effects of Level 3 haven't happened yet.
Is it only that one species of fish that's having a problem?
You might want to look at
this post, which lists the reasons that fish lose health and die. Maybe you'll see something there that will help?
