What is the ethical justification for pescetarianism? & Some other questions.

I'm myself a vegetarian, eating dairy and eggs. I abandoned meat eating due to ethical and health reasons.

I was first vegan for years, but I couldn't maintain it primarily due to being constantly hungry and never feeling satisfied (6'2" 230 lbs), even though I actually gained weight (yes, I'm aware of whole foods, fiber, etc.). I also highly disliked how cooking intensive it is, but that's another topic.

I switched to vegetarianism, and things like quark / cottage cheese and eggs helped somewhat. I was thinking of switching back to meat eating, but I'm quite disgusted by the whole practice (the eggs I buy are sold by local farmers), and the ethical arguments are very solid against it.

Recently I've heard that ancient vegetarians (such as among the Greeks) did not consider fish to belong in the category of meat, which made me look up pescetarianism. Fish also represent a grey area among Indian vegetarians, with some eating it and some considering it impermissible (on the grounds of religion).

I'm interested in how pescetarians would defend their ethics, and on what grounds they consider the eating of fish as permissible but not the eating of other meat (like chicken, cow, etc.). I'd also like some insights on the health front; since vegans mention the heavy metals in fish.