Peta does sometimes go way beyond extreme. And they tend to be overdramatic sometimes. I am certainly against all the cruelty farm animals receive and other cruelty issues like puppy mills. But I think fish can be happy in aquariums and it is all about knowing your fish and being responsible about them. Not keeping large fish in too small a tank, doing water changes frequently, feeding them correctly, ect...
You could make a big deal out of everything. You could say dogs should not be kept as pets because they can't go out and hunt and live in packs as they do in the wild. While dogs have had much more time to adapt to humans than fish and are more capable, you get my point...