It is all relative. Importers holding fish in Quarantine or breeders raising heaps of young will stock up their tanks extremely heavily and get away with it. They generally have bare or at least unplanted tanks, heavy aeration and very frequent water changes. I raise killies in bare tanks with no filtration or aeration successfully. I don't cycle the tank before puting fish in because there is nothing to cycle, feed very heavily and remove excess food frequently, and do very frequent water changes. When they are about half grown they get moved to a more balanced situation in a lower stocked and heavily planted tank (with low filtration and no aeration). If a bare tank was very heavily stocked I would put a sponge filter in there.