Fish based is a great idea!
Love Fromm Surf & Turf. Another one is Wellness Core Ocean. If you have a Costco membership, they also sell a fish-based grain free food too (and it's cheap!).
With fosters we did a switch over about a week and a half. 3/4 old food, 1/4 new food, then up/lower the foods until you're at the new food 100%. You won't get diarrhea this way.