Fish is a perfectly acceptable (great, in fact) protein to feed. Our large dogs are on Acana Pacifica, and little dogs are on ZiwiPeak Venison & Fish with excellent results all around. There is no reason you can't feed a 100% fish diet. In fact, we would all be healthier if we ate mostly fish.
I am not sure why people in general are so uncomfortable with fish as a complete protein source. This goes for people and dogs. The protein is great, the fat ratios are PERFECT (in wild caught, always avoid farmed) and it has a far better nutritional profile than the chicken that makes up most dogs (and peoples') diets.