Allow us to increase quality of fish inside closed water bodies (with no connection to river or any larger water body).
A new structure (Production & Processing), called aquarium maybe? where You can throw fish with some kind of food that will allow it to rise in quality, then after sometime (few days or week), dump the content of aquarium into closed water body, increasing quality of fish inside of it for a month. If there was nothing done to it for a month, quality starts to detoriate back to it original quality unless, each week someone try to increase (or decrease) quality again.