Minimum Tank Size: 55 Gallons (75 Gallons recommended)
Care Level: Moderately Difficult
Water Conditions: 6.5-7.5 (Soft to Hard)
Temperature: 24-30 °C (75–86 °F)
Maximum Size: 13 inches (33 cm)
The oscar (Astronotus ocellatus) also known as the tiger oscar, is large fish that has long been a staple of the aquarium trade. Originally found in South America, it can now be found throughout the world, with large self-sustaining populations in China, Australia and the United States.
In the wild, oscars are mainly found in slow moving rivers, though some have been found in fast moving, white water environments. In these environments, the oscars will often shelter in riffles or near submerged driftwood.
Oscars are by no means a small fish, and some can reach a staggering 18 inches in a home aquarium. With that being said, a far more common size is 11-12 inches. This smaller size can be caused by several factors, but most oscars suffer from stunted growth from being kept in cramped conditions for most off their life.
Not only are they large, but they can also live up to two decades in captivity. The commitment that oscars require should not be underestimated, and even ones kept in less than optimal conditions will usually live for at least a decade. So be prepared to shell out a significant amount of money and time if you are considering purchasing an oscar.
Oscars are often referred to in the aquarium hobby as “a footballs with fins”, and this isn’t an exaggeration. Before purchasing an oscar, it’s important to understand that oscars quickly grow to aquarium busting sizes. It’s not uncommon for a well fed oscar to average one inch of growth a month, for the first year of its life.
Because of their large size, oscars require at the bare minimum a 55 gallon tank, with 75 gallons being a more realistic minimum size. While they can be kept in a smaller tank while they are young, it should never be viewed as permanent solution and should be upgraded to a larger tank size at the earliest opportunity. If you are planning on keeping a pair of oscars, then the minimum tank size quickly increases to 75 gallons, with 100 gallons plus usually a better choice.
Any fish that are kept with an oscar needs to both be large enough and aggressive enough that they won’t end up bullied, or worse – in the oscar’s belly. A good rule of thumb to follow is that any fish kept with oscars should be at least five inches in length. Any smaller, and they will likely fit into a mature oscar’s mouth.
Some of the best choices for oscar tankmates are some of the larger catfish, silver dollar fish and many of the semi-aggresive cichlids, like convict or earth eater cichlids. But while some fish may peacefully coexist with one oscar, another oscar may relentlessly attack the fish and even kill it. Each oscar has its own personality, and you should take care when first introducing any new fish into its tank.
It may be hard to believe, but oscars can also be the victims of bullying and should never be kept with any aggressive, larger fish. Some fish to avoid are flowerhorn ciclids, Jack Dempsey’s and nearly any large African cichlids. As a rule, oscars aren’t overly aggressive, and you should watch any large tankmates to ensure that they aren’t attacking your oscar.
When choosing a filter for an oscar, you need a powerful, high quality filter. Oscars are the definition of messy fish, and their tank water must be kept stable and very clean. If their water quality starts to suffers, it usually leads to illness, including the dreaded “hole in the head” disease for oscars.
Easily the best choice for an oscar tank is a canister filter, but a hang-on-back filter is a close second choice. The key is to ensure that the aquarium is heavily over filtered, as you want the water to remain as close to perfect as possible. Many people actually use two hang-on-back filters to ensure that tank is always kept sparkling clean. Another trick is use to a sponge filter in conjunction with a hang-on-back filter, which provides extra biological filtration.
I would strongly recommend choosing an Aquaclear Power Filter for an oscar tank. This filter combines excellent filtration with a durable design, and it will keep your tank sparkling clear for years to come. You can also read the Aquarium Tidings Aquaclear Filter Review here.
Oscars are a prime example of the saying “eyes are bigger than its stomach”. They will attempt to eat anything that they can fit in their mouth – even if they can’t swallow it. In the wild, their diet consists mainly of small fish, invertebrates, insects and fruit. It is very important to replicate this diet in the home aquarium, and that includes providing them fruit on a regular basis.
While the list of what oscars can be fed in the home aquarium could make an article on its own, the best way to replicate their natural diet is to provide a high quality cichlid pellet or sticks, along with frozen foods, fresh-cut fruit and live food.
It’s important to avoid the temptation to feed them human foods, as these are usually very high in fat. Their diet should be restricted to food available in the fish store and any of the many specialized recipes available online. They will greedily accept frozen krill, bloodworms, daphnia and blackworms, and these provide important nutrients to the oscar.
Providing live foods is considerably more difficult, and no matter what you may hear, you should never feed oscars feeder goldfish or rosy red minnows. These fish contain thiaminase which binds to vitamin B1 and will cause a dangerous deficiency in oscars. Other live fish can be fed to oscars, but it is imperative to ensure that they are completely healthy. Most feeder fish found in stores that oscars can safely eat (guppies, mosquitofish) are usually riddled with parasites and disease.
The breeding of oscars is relatively easy – if you have a pair that will mate. In fact, the most difficult aspect of breeding oscars is finding that elusive bonded pair. The only way to accomplish this is to either purchase a pair that have successfully bred in the past, or to purchase six oscars, and let them pair off in the home aquarium.
To further complicate matters, it’s almost impossible to sex oscars. There are all sorts of articles online telling you little ways that you can “probably” tell what sex an oscar is, but until you see the eggs in the aquarium, you can never be sure.
In order to have six oscars in the home aquarium, you’re going to need a massive tank, and 200 gallons is usually the minimum size for six adult oscars. And once the oscars have paired off, you must remove all other fish from the tank. Breeding oscars become extremely aggressive, and will terrorize any other fish in the aquarium – including other oscars in the tank with them.
Once the two oscars have successfully paired off, you need to be sure that the water in the tank closely matches their natural habitat, which is soft and acidic. They will have difficulty breeding if the water is too hard or if the pH is too high. However, unless you know truly what you’re doing, it can be incredibly dangerous for your fish to alter the water chemistry. I can’t stress this enough – it’s better to have water that is too hard and basic, rather than have pH spikes and unstable water chemistry.
Once the water chemistry has been dealt with, the oscars must be provided with a large, flat surface to lay their eggs on. If you don’t provide them with a surface similar to this, they will often remove all of the substrate from one side of the tank, and actually lay their eggs on the flat glass at the bottom. A pair of oscars cleaning a flat surface is one of the surest methods to identify early breeding behavior.
Something else to be aware of is that the courtship behavior of oscars can be extremely violent. It’s not unheard of for one of the fish to die from their injuries, even before any of eggs have even been laid. If one of your fish looks to be badly injured, remove it to a hospital tank and treat it with antibiotics. Once it has fully healed, you can try again.
If the oscars are successful in their courtship, the female will move to a flat surface and deposit her eggs, while the male fertilizes them. Don’t get disheartened if the oscars are seen eating some of the eggs, as they will often remove any infertile or bad eggs.
Even if the oscars eat all the eggs, or eat the fry right after they are born, don’t be too concerned. Oscars often need at least one attempt at parenting before they get it right, and the next time that they breed they will usually be more successful.
Once the fry have hatched, they can be feed baby brine shrimp, or any of the commercially available fry food products. I have used Within 48 hours, the eggs will begin to hatch, and the newborn fry can be fed with infusoria, baby brine shrimp or any of the commercial fry foods. After a few weeks, the fry can usually be fed finely ground flake food.