I seem to notice that when I have 15, 20 fish in a tank they seem to have some sort of illness every time I open the program. I have fully researched food and fish environment. I bought all 4 magic fish, the snail, and the seahorse. I also have 5 magic fish in the tanks. I never see any dead fish and I don't over feed. Is there something else I'm not noticing or is my theory correct and fully tanks are more susceptible to illness