aquariums are initially empty and you get the fish from fishing/Markets and the bigger the aquiarium the bigger the capacity to have more fishies.. or a bigger fish
small aquarium = 3 small/1 medium fish
medium aq. = 5 small/2 medium fish
large aq = 10 small/5 medium/1 Large
and you can mix small medium fish playing with available fish capacity
HQ fishes look different and more colorful/shinier than NQ fish!