Some people love them, others hate them, either way you just can't ignore them. Of all the fish I'm asked about, the Oscar must be the most common. Oscars, Astronotus ocellatus, can hardly be called pretty fish. In fact a better description might be, "as ugly as sin." Yet after more years in the aquarium hobby than most fishkeepers, they still remain extremely popular among both old hands and beginners alike. There are even some fishkeepers who keep nothing but Oscars.
Why? Well the best reason I can think of is attitude, and Oscars have it by the bucket load. No other fish has such a personality, seems so intelligent and can attract your attention so well. Oscars will quickly learn the hand that feeds them and will react to their owner entering the room, swimming up and down at the front of the tank almost begging for attention or more commonly, food. This isn't a simple reflex action, Oscars will studiously ignore anyone else.
The Oscar originates from the Amazon river basin, although it is true to say that man has introduced them elsewhere. A natural predator of smaller fish, the Oscar is hampered by its lack of agility and so has developed a very interesting method of hunting: hiding in weed beds and floating plants while remaining very still, until some poor unfortunate creature swims just a little bit too close and then the Oscar lunges, swallowing its victim whole.
Oscars require a big tank, 48" x 15" x 12" is the absolute minimum for just one Oscar. If you would like a pair then 48" x 15" x 18" is required. You should bear in mind that a small 4" fish will only take 12 months be be a massive 9" fish and so if you cannot provide a big enough tank to start with then don't buy Oscars. Promises that you will get a bigger tank later never seem to become a reality. Besides, you will need that bigger tank in less than a year, so it is best to wait until you can afford it and get the Oscars then.
As far as tank mates go, choose something as large as the Oscar itself. Despite their reputation Oscars will not be able to cope with large territorial Cichlasoma. South American cichlids like the larger Geophagines are milder in temperament and can be kept successfully with a single Oscar. If you intend to keep a pair, they will need a tank to themselves; if they decide to breed then all tank mates will be seen as a threat. I have seen a number of recommendations that armored catfish can be kept with a pair of Oscars, "to clean up the tank floor". While this is undoubtedly true, I can't personally recommend it. Cichlids have very poor night vision and will be unable to defend their eggs and fry against the nocturnal predations of a catfish, unless of course a light is left on. Proper aquarium maintenance is, in my opinion, a far better way to keep the aquarium clean.
Decor for an Oscar tank should be kept at a minimum. A few rocks and maybe a piece of bogwood should be more than sufficient. A pair of Oscars will also need a couple of large flat stones on which to lay their eggs. A piece of slate is best. Make sure that these are at least 8" x 10" as Oscars produce an enormous number of eggs. Heaters should be either protected by a heater guard or hidden where the fish cannot get to them. It is not unknown for Oscars to attack aquarium equipment, particularly heaterstats. I don't really think the species have anything against heaters generally but most of these are reflective and I think that the fish is attacking its own reflection in the mistaken belief that it is another fish. Plants are best avoided; Oscars will treat them as expensive toys and constantly rip them up.
Filtration is one area where a number of otherwise perfect Oscar set-ups can fail. Oscars are extremely messy eaters and so good filtration is essential. While many prefer under-gravel filters, it needs to be remembered that Oscars are great diggers and should you adopt this method then a gravel tidy is a must. My own personal recommendation however, would be to use a large external power filter filled with sintered glass or even better, Siporax. Siporax, while expensive, has the distinct advantage that it will eventually develop some ability at de-nitrification and help reduce the ever present nitrate problem inherent in a tank without live plants. For the same reason, a good activated carbon or nitrate absorbing resin should also be added to the filter. An internal power filter can also be added to provide mechanical filtration and help keep the water polished. Oscars need plenty of oxygen so set these filters so that they create a disturbance at the water's surface or add some aeration. Despite their Amazon origins Oscars can be kept in hard as well as soft water providing it is kept clean. Temperature should be in the range 26°- 30°C. Weekly 20% water changes will help keep your water quality good.
One last thing that needs to be add to any Oscar tank is something for the fishes to play with. Oscars just love toys; a ping-pong ball floated on the surface will provide the fish, and you, with many hours of enjoyment.
When it comes to buying the fish, think very carefully about which color morph you really want. The classic Tiger Oscars will lose a lot of their tiger pattern as they grow. Red or even partial albino tiger Oscars are available. You're going to be looking at these fish for a long time so make sure that you get the colour morph you really like.
If you intend keeping a pair of Oscars then select four young fish, ideally from different sources to reduce the chance that they are related. These fish can be up to 4", the reason that you will need four is simply that Oscars are very difficult to sex and getting two adults to live together without fighting is a task that would have defeated Hercules himself.
By the time the fish are about 7" in length a pair should form and start to give the other two fish some really serious aggravation. At this point you will need to move the spare two to other accommodation or pass them on. Oscars can take up to two years to become sexually mature, so don't worry if the pair doesn't spawn for a while.
When the Oscars do decide to spawn, they begin displaying and cleaning a piece of slate. This can go on for a number of weeks. Displaying involves a lot of tail wagging and can lead to a session of slate cleaning or tail slapping, both fish swimming either side by side or nose to tail. These incidents can become quite violent and involve mouth wrestling and biting. Don't worry about this or the injuries that the fishes will cause each other as this seems to be a fundamental part of the mating process. Perhaps each fish is testing the other's suitability as a mate. Very rarely one fish will get trapped in a corner of the tank and take a real beating. If this happens you might need to intervene but it hardly ever does.
As spawning gets closer the intensity of displaying will increase and a lot of digging will take place. It is now that the female's creamy egg tube will appear. When you see the tube, spawning is only about eight hours away.
After all the activity of the build up, spawning itself is a pretty mild affair. The female makes a pass across the slate, laying a row of about twenty adhesive eggs. The male follows behind her gently fertilizing them. This is repeated many times until between one and two thousand eggs are laid. If this is their first spawning, the parents may eat their eggs. Don't worry about this. It is a result of immaturity and they will soon spawn again.
The parents will defend the grey-white eggs and the fry will hatch in about three days. For the first six days of their lives the fry will be absorbing their yolk sacs and will not need feeding. The parents will move the fry to pre-dug pits by taking them up in their mouths and spitting them. When the fry become free swimming they will need lots of food. Finely sieved flake or even better, newly hatched brine shrimp, should be fed three times a day. Young Oscar fry may also feed off their parents' mucus like some other South American Cichlids, most notably Discus.
The fry will grow very quickly and need thinning out. Don't try to rear them all. In nature only one or two would survive. As fish breeders we should not try to raise sub-standard fish and hence dilute the integrity of the Oscar gene pool. You will also find it easier to sell a dozen quality Oscars than a thousand stunted ones.
The fry can stay with their parents until they are about an inch long. At this point the adults are very likely to spawn again and at that point, the apple of their little fishy eyes will quickly become the lining of the massive fishy stomachs.
When considering what to feed an Oscar it is very important to remember that Oscars can get hooked on one food and just refuse to eat anything else. Because of this feed Oscars something different everyday. Cichlid pellets and frozen foods such as lance fish and whitebait are very good. Frozen and live river shrimps are readily accepted. One of the best Oscar foods are excess fry from other cichlid breedings and of course the good old earthworm. I have heard a number of people suggest the raising of guppies/mollies/goldfish etc. to feed their Oscars. In my experience however, the effort required to breed these fish, in both sufficient size and number is far in excess of any benefit gained.