Lungfish

One species, three tiers: the South American gold spot lungfish, Lepidosiren paradoxa. Its gills are reduced to the point that it breathes air obligately — deny it the surface and it drowns, which makes lid clearance matter more than filtration horsepower. Buy it if you want a fish that recognizes you, takes food from tongs, and stays with you a long time.

It bites, and it does not distinguish between food and a hand. Adults are strong enough to injure, so plan on a tank with no tankmates, a lid that latches, and tongs rather than fingers at feeding time. Growth is slow but relentless; buy the footprint now rather than promising yourself an upgrade later. If it is shipping rather than being picked up, read the livestock guarantee first.