Black Arowana

Almost nobody should buy this fish. Osteoglossum ferreirai grows to around 80 cm in captivity, and the published maxima run larger: FishBase gives a total length of 90 centimetres, and the IUCN account summarised on Wikipedia mentions individuals to 1.2 metres. The 80 figure on this page is an editorial estimate of a well-kept adult rather than a published measurement, because neither source lists a standard length for the species.

The black arowana belongs to the Rio Negro basin, including the Branco, in blackwater that FishBase records at 24-30 °C. Populations found in the Orinoco in the 1970s are still argued over as natural or introduced. Juveniles are easy to identify, black with yellow bars along the body and fins, and they lose the pattern at around 15 centimetres as the dark steel adult colour comes in.

It hunts at the surface with an upturned, drawbridge mouth, taking shrimp, insects and smaller fish, and it jumps at prey, so a heavy gap-free cover is not optional. Males mouthbrood as many as 210 eggs and release the fry at about seven centimetres.

The stocking numbers here are ours, not a source's. Neither FishBase nor the IUCN assessment publishes a pH range or a minimum volume for the species, so this page lists pH 5.0-6.5 to reflect its blackwater origin and 800 litres as a floor for a single adult, which means a purpose-built tank rather than anything sold in a shop. Keep it alone.