Actually our forefathers evolved around the great lakes in Africa (a lot of fat fish there). That's why fish is so healthy for us. Fat fish is actually good because of the healthy fats. Dont skip all fats, eats some nuts, olive oil and fat fish now and then (maybe some fish oil). It'll even help you lose weight.
Switch it up if you're worried about mercury (or dioxin or whatever) poisening. A mix of healthy fat fish, egg whites (with maybe a yolk now and then), chicken, tuna, and maybe some fat-free (or skim) milk or cottage cheese (like an hour, maybe 2 before you go to bed). Whey should probably be kept for after workouts only. It doesnt offer many benefits on other parts of the day. I dont see why you would limit yourself to those two you mentioned.
Dont you have some heart (or cardiovascular?) problems? I remember that you had some sort of problem, what was that about? I recently read that a diet too high in protein causes the arteries to stiffen somewhat. So don't overdo it.