As a vegetarian mostly for environmental reasons, I get frustrated when someone does the, "but you will eat fish, right?" Uh....no. Environmentally speaking, pretty darn bad, particularly, as SGTiger stated, if it is farmed.

That said, I've been keeping an eye on some of the work on closed-circuit fish farming case studies. They're taking old warehouses and creating closed environments for freshwater fish that include water-treatment via plants, including herbs and salad greens.