Oscars, cichlids that have superpowers.

Posted by Ruth McDonald on

Astronotus ocellatus

Oscars, the brown and orange fish that you see in so many tanks. Often with misshapen faces and arched backs, rather than a streamlined block of muscle and often anger. There are as many myths about these guys as there are facts that are correct. But Oscars in my opinion are one of those amazing fish once you scratch under the surface.

There is a patchy history in the information about Oscars. The first illustrations of Oscars seem to be from preserved specimens, or descriptions. The teeth and the colours of the fish are wrong, even taking into account how drab wild Oscars are in comparison to the aquarium morphs there is still a lot of missing details. But the worst thing is that the habitat is listed as the Atlantic Ocean. Not the Atlantic coastal region or the Atlantic area, they are listed as a marine fish.

We have however done a lot more research since then and found out some amazing facts. But here is where things start to get a little hazy. There has been some suggestion that the Oscars we have in the pet trade, and those that have been released into the water ways of north America are a different species. There is also second species of Astronotus, the wonderfully named Fat Oscar, Astronotus crassipinnis, carassipinnis literally means Fat Fish. Do we have a different species, a hybrid, or is it just that up to a hundred years of collecting this fish for the aquarium trade has allowed some major differences to form, maybe even the species A. ocellatus is made up of individuals that are different from each other.

But none of these are super powers, but there is some thing that is. When the water levels start to drop and the oxygen levels in the water start to get low, Oscars are adapted to be able to survive in water that other fish would suffocate in.

Oscars are one of the many species of fish that guard their nest. In fact they go far beyond that. Oscars can breed up to 4 times in a single year, and a large female can lay up to 3000 eggs at a time. Which is one of the many reasons Oscars are so good at being an invasive species. But parental care doesn’t stop there. One the eggs are laid and fertilised the female will mouth the eggs clearing them of dirt and probably coating them in an antibacterial mucus. At the same time the male will defend the nest area from predators and keep other males away from his female.

Once the majority of the fry get themselves out the the egg the parents will move them to a new location. This is probably to leave behind the unfertilised eggs that may now be starting to attract bacteria and fungus. Over the next 8 to 14 months the parents will gradually raise the fry until the youngsters are old enough to go off and breed themselves and start the cycle all over again. Some pairs have been seen to carry on breeding even if they are looking after young, but no one is sure if they raise multiple batches of youngsters.

The issue with nest guarding is that for at least a week the parents have to stay with the nest. Once the fry are moving about both will stay with the fry, grabbing any young in their mouths and spitting them back into the group. These nests are normally in the flooded forest either side of the river channel. Here the nests are safer, the predators are easier to keep away, but the oxygen and food is a lot lower.

It turns out Oscars can stand low and very low oxygen levels, and a week without food a few times a year isn’t that much of an issue. In fact if you starve them first they can cope better with low oxygen. But no this doesn’t mean that you can keep them in poor conditions. Come on people.


Oscars are native to South America including Orinoco and Amazon basins, French Guiana, and northern part of Paraguay drainage, Parana basin (Kullander 1986). Sadly they are an invasive species in many places around the world, most notably North America.

Water conditions

Despite their South american origins, Oscars are suprisingly adaptable, Aim for a ph between 6 and 8, keep the water temperature between 22 and 25 degrees C, and keep the hardness in the 5-15 DH sort of area.

Tank Size and conditions

You’ll often find sources saying they need to be in a minimum of a 120 cm tank. This is a bare minimum for growing on fish. They routinely get to 35cm long and they are messy fish that are big for their length. I keep my pair in a 750l tank 210 cm x 60 cm x 60 cm and honestly for an adult pair I wouldn’t go much smaller. Give them space and you will be rewarded with some great pet fish that you can train to do a lot of behaviours.
Sadly you will see stunted Oscars far too often in aquaria, this is probably due to hormones and excess nitrates. Just because they can survive poor water quality doesn’t mean they will thrive. These guys need large regular water changes. I do about 80% twice a week.

Tank Mates

Here’s where you get issues, Oscars emit a smell that to a lot of other fish, smells like a predator. So that suddenly limits your options. Larger plecs, and other big fish can get on with them, but some individuals are just bloody minded and you may end up with one having to live on it’s own.


As nice as they look you keep oscars for their behaviour. They’re intelligent, sensitive souls that will throw a strop at the slightest change. Treat them as the spolit brats they can act like and all wil be fine, and by this I mean keep the tank exactly as they want it. Do plenty of things to keep them occupied, and make sure their meals are varied and interesting.
In the wild Oscars are both predators of smaller fish, and prey for other fish. The eye spots on the tails help them avoid being bitten by certain species that include piranha. I’ve observed them hiding from snakes indicating they recognise that particualr shpae or momevement as being something that might eat them, Don’t forget their ancestors may have lived alongside anacodas.


Yeah they’re a pain here as well Oscars need vitamin C in their diet, so you will need to feed them fruit and veg as they would eat in the wild, or if they are stubbord feed live food such as meal worm and dubia roaches fruit, and then feed them to your fish. They are recrded in different studies as eating fruit, smaller fish, insect larvae, insects, vegitation, snails, shrimp, clams, and general detritus. Make sure you offer a good variety and ensure they’re getting enough vitamin C rich foods, and they will thrive. They do have carnivourous tendancies, so aim for more higher protein foods. In general feeding smaller amounts a few times a day will help with keeping water quality to a higher standard as they are messy, messy eaters.

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