I don't know if this is a bug or not, but I certaintly don't understand the logic of this:

I have the fish of curing in tanks 1 &2, the seahorse in tank 1 and both food and environment levels researched.

I left the game alone for 12 hours and come back and many fish in tanks 1 & 2 are sick....but the sale tank, where there's no fish of curing or seahorse, they are all fine. And all tanks have the same amount of fish in them...I don't get it.