Threadfin Acara, Acarichthys heckelii

Common Names: Threadfin acara, threadfin cichlid, Heckel's cichlid, "fairy cichlid"

Scientific Name: Acarcichthys heckelii (Muller and Troschel, 1849). Sometimes referred to as Geophagus thayeri.

From Where? South America, across most of the Amazon and the main surrounding rivers, like the Xingu and Negro, as well some other arbitrary rivers thereabouts as well - pretty much wherever there are tropical rivers.

Size: Gets to a maximum of 15cm (6 inches), and captive specimens can reach this size if given proper care.

Foodstuffs: Will accept a huge range of foods, but don't take this for granted. You'll only get the best growth and colour out of these fish if you supplement their diet with frozen/live foods. Some people recommend adding some veggies to their diet, which makes sense if you want to give them an all-round meal.

Water: Try to keep the water similar to what you'd keep your 'typical' Amazonian fish in - a lowish pH (6.0 - 7.0, although they're tolerant of slightly higher), and kinda soft. You won't want to be adding any buffers or salts to you tank, or rocks that may dissolve and alter the water chemistry.
They're proper tropical fish and appreciate it when you keep the water between 24° and 28°C (75° to 82° F); raising the temperature is said to induce spawning.

Pretty fishy

Aquarium Specifics: Not particularly fussy. A three foot (90cm) tank can work for a group of 2 or 3 (1 male and a female or 2), and the decor should be somewhat 'foresty', but that definition is really up to you. Occasionally they'll make use of hiding places, and wood in the tank helps keep the pH lower, so putting 2 and 2 together, wood sounds like a good idea. The aquarium lighting shouldn't be too harsh, but not so dim either. Try to strike a balance between what suits you and what suits your Threadfin.

Compatibility: They aren't bad characters, really. Even the males don't cause any noticeable chaos, although I wouldn't put this to the test by cramming too many males in a small tank with insufficient females. They'll get on with small to medium peaceful fish, so long as these fish aren't noted fin-nippers, like tiger barbs, or general bad-boy bullies, such as some cichlids, especially those that go nuts during breeding (convicts, jewels... )

Specific Problems: Especially at first, they're susceptible to some diseases, mostly parasites. Watch for signs of external parasites (scratching against gravel or decor) and internal parasites (not growing or gaining weight, concave belly).

Rarity: Can be hard to get your hands on, but if you know the right places to go to you'll be able to find some.

Similar Species: sometimes gets muddled up with Geophagus (earth eaters) or Mikrogeophagus (Ram cichlids), especially when younger. It's useful to carry around some form of identification if you're looking for them.