This is exceptionally cool – these are captive-bred Black Ghost Knifefish, Apteronotus albifrons, spawned and reared in captivity. For a fish that was already considered pretty hardy, these should be even easier! Of course, it bears repeating that while these are small and incredibly cute (check out the image with a 5″ sponge filter for size comparison), they have the capacity to grow to 18-20″ in length, which necessitates an aquarium large enough that they can turn around (opinions vary, but I think just from a “turning around” standpoint, you need an aquarium at least 18″ front to back). These captive bred juveniles will eat everything, even flake, although they definitely prefer meaty foods. When first made available, they are sold at a small size, roughly 3-4″ inches in length. Medium (5-6″) or Larger fish are generally fish that have been grown out.
While people have had success recently spawning and rearing them even in home-based settings, the fish need to be mature (very large) and compatible (reportedly very difficult). Even just watching two of these captive-bred fishes in a 75-gallon tank…they’re not exactly best of friends and prefer to remain solitary. Obviously, they are held in large quantities in wholesale settings, but unless you have a very large aquarium, or multiple aquariums, and have your sights set on trying to breed them someday, I’d probably suggest only purchasing one per tank.
Note; in the freshwater aquarium trade, the term “tank-raised” is often used to denote fishes which were bred in captivity, and not wild-caught. However, “tank-raised” has a different meaning in the saltwater aquarium trade, so I am using the term “captive-bred” which is more concise.