Priority 1) Go for the Boxfish first. Do the Ask for help.
Priority 2) Go for Gobies
Priority 3) Go for Clownfish
Priority 4) Go for Butterflyfish
As you can see from the pictures here, the Fish Mastery gives more XP for these families (Butterflyfish, Clownfish, Gobies, Boxfish) than other type of fishes from the Store.
Within these 4 families there are certain families which gives more XP than others. In that Order I just listed.
Just follow my Priorities and you will be sure to get your Fish Mastery in no time :)
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Share and learn. Learn and share :)