Actually it has has been established that dogs thrive on veg*n diets. One of the oldest dogs in the world is a vegetarian.
This is not intended to humanize dogs. It is intended to reduce unneccesary suffering of animals. Yes it's not natural but then nether is keeping a dog in a person's house, giving them vet care or many other things which are done to dogs in captivity.
Dogs are more on the omnivore side of things so this may be why it works out nicely.
PETA does euthanize, they also do some rescue work as well. Yes PETA specializes in shock tactics. Some of which I am not fond of. If you wish to become informed about PETA, hopefully you will read both positive and negative viewpoints.
Overall I'm guessing PETA is not going to be popular on a site like this just because they are against fishkeeping.
PS: I'm veg too and former PETA member.