Interesting topic, for sure.
Natural Balance LID Fish & Sweet Potato has only 10% fat, and not as many preservatives as other foods. California Natural 11% fat (but was just bought by Proctor and Gamble, yuck).
Obviously if you have looked into dog foods heavily, you know that a lot of people feed a raw diet instead of any kibble. I do think this is the way to go-especially if you can feed whole fish to your carnivores, raw plants and veggies to your herbivores, etc. The problem with fish is that every species is slightly different...ranging from straight piscavores, to straight filter feeders, and everything in between. I don't think much testing has been done on the specific nutritional needs of different types of fish..so it's hard to know if you are getting it right.