PLA really isn't suited for water, as it will degrade over time. Maybe not by a "my part is broken" amount, but at least it dissolves a little bit into the water. It's probably basically just a bio-pellet.
ABS is a complete pain to print, but there are various stories of printed parts leeching various things.
PETG is basically food safe, more or less prints well (it's a little fiddly, but printable on an open frame with some adjustments for speed/etc) has good heat resistance, and is just generally a decent material. For any aquarium part, I kinda see no reason to use any other material.
I guess the downside to PETG is it's a little more brittle than PLA sometimes, and its usually more stringy.. so.. eh..