Anyone know of any fish based puppy foods?

I'm having trouble with my adult Shih Tzu and Boston Terrier puppy. My Boston Terrier puppy will not leave my Shih Tzu's food alone! At the moment she is eating a mixture of IAMS Premium Protection puppy food and Wellness Just for Puppy. I am transitioning her over to the Wellness since she hates the IAMS but I want to do so gradually to avoid any stomach upset. She enjoys the Wellness more than the IAMS but it still not 100% satisfied with her food as she is constantly stealing food out of my Shih Tzu's bowl. He eats a mixture of Wellness Core Ocean and Acana Pacifica and she INHALES it! I've never seen a dog eat a food with such gusto! I free feed the Shih Tzu and have been doing so for the past five years and this method works great for him. He is very healthy and in shape. But due to this way of feeding it is easy for my puppy to get into his food. Should I change my dog over to a fish based food since she seems to love the fish or stop free feeding my dog? All suggestions for solutions to this problem would be greatly appreciated. Thank you! :)