Free your Fish!

I am very much a collector or hoarder by nature, sentimental in the extreme, and I therefore find my tanks filled to capacity at all times, mainly with the gorgeous Mermaids one can breed at Level 15 and higher.  I do see, however, that if you wish to increase your Levels, selling fish is the way to go.  You do not earn the XP by breeding them but by SELLING them.