The Short Answer
Vast quantities of fish, vast quantities of veggies. Vast quantities of milk if your pescetarianism allows dairy. Vast quantities of eggs if your pescetarianism allows eggs. Plenty of olive oil on the salads (and the eggs, and the veggies). Be careful of too much mercury in the fish.
The Longer Answer
If avoiding factory-farmed meat is your primary reason for eating veggies and fish, I suggest grass-fed, free-range beef. For one, grass-fed cows are practically a different animal from corn-fed cows, and the grass-fed nutritional profile of their meat and milk is much better.
I am very partial to meat shares for getting humane, non-factory meat. I get a quarter of a cow (enough to half-fill its own freezer), including ground beef and a wide variety of cuts (sirloin and kidneys and flank steaks and roasts...), all for a reasonable per-pound price. Split it with a friend or two if the up-front cost is too great, or if you don't have the freezer space.
If you can find it, milk from non-factory/grass-fed cows is an awesome part of a mass-building diet. Milk is nutritionally dense and complete, and so adding a half-gallon to your three big meals is a simple way to add healthful calories.
Maybe It's Not The Diet
Your issue might be diet, but it might also be that the exercises you do aren't ideal for adding mass. Compound exercises are usually better for adding mass than bodybuilding in many circumstances (see this question). Your question is specific to pescetarianism, but you would find good general advice here. Don't forget to read up on conflicting opinions of GOMAD.
On a side note, you're drinking your protein shake right after working out, right?