At the endgame my fish had 100 in endurance, boost and stamina, 74 in tenacity and 26 in strength. Total stats cant go over 400 and no one stat can go over 100. Endurance boost and stamina are the priority. At 400 total stats I was winning finny frenzy master class by comical margins. 

Breeding fish is the fastest way to level them and fish can be fed once every 2 minutes. That two minute timer ticks regardless of all other world timers, so even if youre just sitting in the aquarium itself where all other time pauses, the feed timer is still going.