The Reigning Titan:

Queensland Lungfish, Neoceratodus forsteri. If you’re a lungfish fan already, you’ve probably heard of ‘Granddad’, a Queensland Lungfish living at Chicago's Shedd Aquarium since 1933, who only recently passed away, well into his 90s. At the time, that made it the oldest living fish in captivity! Even with the best medical care, we humans are lucky to live to an age that ripe, but for Neoceratodus such longevity isn't uncommon — his former tankmates are already catching up. Queensland

Lungfish can reach an incredible weight of over 40kg, putting them safely in the freshwater megafauna camp. In the hobby, they are now highly sought after. As with many megafauna, the IUCN lists them as endangered, with their remaining populations needing protection. You can only purchase captive bred specimens, and only two farms are currently licenced to breed them.