If your aquarium is attacked by normal Clash or Aphrodite Clash, and some of your fish have already fulfilled 2 times the ROI. It will have a chance to get sick, which means the fish can't generate more rewards while it's sick.
How do you fix this?
You must give medicine to the associated fish. The amount of medicines is directly related to the rarity as it is in the following table:
Amount of medicine
Medicines can be found in the shop section.