Oscar

Astronotus ocellatus

Also known as: Tiger oscar, Velvet cichlid, Marble cichlid

Water Parameters

• Temperature: 23–28°C
• pH: 6–7.5
• Hardness (GH): 89–268 ppm (5–15 dGH)

Values above are general hardness (GH). For carbonate hardness (KH), the buffer that keeps pH stable, see the water parameters guide.

Care Summary

A large, intelligent South American cichlid sold as an appealing 5cm juvenile that becomes a 30 to 40cm tank-buster within a year or two, which is the entire reason it needs an honest care sheet. Oscars are genuinely characterful, the closest thing the hobby has to a pet with a personality: they learn routines, beg for food, recognise people, and rearrange their decor. They are also enormous waste producers that demand a very large tank (400 litres and up for a single fish), heavy filtration, and frequent water changes, and they are prone to hole-in-the-head disease when diet or water quality slips. Feeding live 'feeder fish' is a common mistake that risks disease and poor nutrition; a varied prepared and frozen diet is far better. Not a community fish and not a starter fish, but an outstanding centrepiece for a keeper who can house one properly.

Tankmates

An oscar's stocking rule is simple: anything small enough to fit in its mouth will eventually be eaten, and an adult oscar's mouth is large. That rules out the entire community-fish range, shrimp, and even mid-sized fish. The realistic options are to keep an oscar alone (they are intelligent, interactive 'wet pets' that recognise their keepers and rearrange their own tanks) or with other large, robust fish in a very big aquarium: large plecos and synodontis for clean-up, or similarly-sized cichlids while accepting the risk of territorial conflict. They redecorate by uprooting plants and shifting substrate, so hardscape must be heavy and secure. Powerful, messy feeders that demand serious filtration and frequent water changes.

Compatible with

• Common Pleco
• Synodontis Catfish

Avoid keeping with

• Betta
• Neon Tetra
• Guppy
• Angelfish
• Bronze Corydoras
• Cherry Shrimp

Common Problems

• outgrowing the tank (reaches 30-40cm)
• very heavy bioload demanding large tanks and strong filtration
• hole-in-the-head disease from poor diet or water quality
• predation on any tankmate small enough to swallow

Sources: seriouslyfish.com, fishbase.org · Last updated: 2026-06-20