Black arowana (osteoglossum ferreirai) have been referred to as "living fossils," because they've changed very little over the last 150 million years. They're native to the Rio Negro Basin in South America, where they live in both black and white water. These large predators can reach up to 40 inches in length, although most kept in a home aquarium will only reach around 30 inches.
Due to their large size, black arowana need to be kept in a big aquarium. An adult should be kept in a tank with a minimum capacity of 240 gallons and a minimum width of 30 inches, although smaller specimens can start out in a smaller tank if necessary. Their tank will require a sand or gravel substrate, store-bought pieces of driftwood and plenty of vegetation. Make sure the lid of your aquarium is fixed securely, as these fish can jump up to six feet out of the water.
As tropical freshwater fish, black arowana need to have the water in their tank heated to between 75 and 85 degrees Fahrenheit. The optimum pH for the water is 6.4, but it can be kept between 5 and 7, and the hardness should be kept between 2 and 4 degrees of carbonate hardness. Their water should be kept as clean as possible, so use a high-powered filter and do a 25 percent water change every week or every other week.
Black arowana are strict carnivores, although they're not especially picky eaters. They can be fed a combination of specialist cichlid or arowana pellets or flakes alongside a range of fresh, frozen or live meaty foods. Suitable foods include minnows, krill, ghost shrimp, large insects, spiders, tubifex worms and mealworms. Younger specimens should be fed two to three times each day, whereas adults should be fed just once a day. Feed only what they can finish in three to five minutes.
Black arowana are generally a peaceful species of fish, although they tend to be aggressive toward members of their own species. They can be kept with other fish, but bear in mind that they'll eat any live fish that will fit in their mouths. As such, you'll need to keep them with other big fish, such as lima shovelnose catfish or large oscar fish. If you're keeping several large fish in one tank, be certain that they have enough room.