The Banjo Catfishes of the family Aspredinidae are a diverse group of South American Catfishes, containing about a dozen genera and fifty plus species. Most of these species and genera are incredibly similar in appearance, and telling them apart requires a very close examination; most imports and offerings of "Banjo Catfish" are a mixed bag.
With few exceptions, Banjo Catfish are similar in their aquarium husbandry. Most species will reach anywhere from about 3" to 4-5", with a few species reaching much larger sizes. Typically found in leaf litter in the wild, they will do quite well in a heavily planted tank. They may also bury themselves in sand. These are an interesting, unusual fish that you may only glimpse from time to time in the aquarium, especially around feeding time.
Care should be taken when choosing tank mates. While most banjo catfish are quite small when sold, they can quickly reach a fair size, and it is worth assuming that they will reach 4-5". They are opportunistic, nocturnal predators, and anything that can fit in their mouths will vanish, without a trace or explanation. As such, do not keep them with small fishes. They are not aggressive, and should not be housed with fish that will pick on them, either.
Large, active danios, rainbowfish, and barbs are all good suggestions, as are other catfishes (e.g., Corydoras and Loricariids), and moderately sized, unaggressive cichlids. These fish are incredibly hardy, and provided your water is reasonably wet, they should do quite well. Ideally, they should be kept in soft, acidic water.