Name: Oscar (Astronatus ocellatus), also known as velvet cichlid, peacock cichlid (NOT peacock bass)
Size: Some people say oscars grow to over 20", but realistically in an aquarium you should plan on an adult size of around 14". Some will be smaller, some will be bigger.
Origin: Oscars come from the amazon river system in South America.
Tank Size: I'd recommend a minimum tank size of 75g (4'x18"x18") for a pair of oscars and a couple of bottom feeders. If you want to keep more oscars you need a bigger tank. Some people say 55g is ok, but I'd recommend the bigger tank. In short, the bigger the tank, the better.
Filtration: My knowledge is not the best in this area, as I have always used trickle filters, and IMO they are the best choice for any tank 75g or bigger. There are many options, but I'd say quantity is more important that what kind. Oscars make lots of waste, and need lots of biological filtration to take care of this. Due to oscars digging habits undergravel filters are practically useless.
Stocking Densities: Oscars are large fish, that make lots of waste (uneaten food and the other stuff). If you crowd several oscars and other large fish into a small tank your fish will not be as healthy, will not grow as fast, will be more aggressive, will probably not live as long or grow as big, and your water qualities will be harder to control. Avoid the temptation to but lots of juveniles, they will grow. Two large healthy fish are better than 4 smaller fish with hith, etc.
Tank mates: There are many fish that can be kept with oscars, depending on your tank size. IMO red bellied pacu's can be kept with oscars, provided you have a large enough tank ( www.fishesrus.com http://welcome.to/thefishcube ). Oscars will do well with most central/south American or west African cichlids, provided they are not too large or aggressive. Some of my favorites are convicts, firemouths, sevrums, jack dempseys, and geophagus juripari. There are MANY more. Many of the large barbs (like tinfoils) make good oscar tankmates, if kept in appropriate numbers. Silver dollars and bala/silver sharks can be kept with oscars, if they are bigger than the oscars when the o's are introduced. Both of these species are schooling fish, so your tank should be big enough for at least 4, plus the oscars. Bottom feeders are a very good idea, because of the large amounts of food oscars drop while eating. Common pleco's are good, tough fish, and will also eat algae off driftwood, rocks and the glass. Their diet needs to be supplemented with algae disks, and vegetables, such as zucchini. Pictus catfish are very active, and make an attractive addition to any oscar tank. They have a max size of around 8" and like any other bottom feeder, they should not be expected to survive on whatever food the oscars miss. Synodontis catfish are the other most commonly kept catfish (especially the featherfin variety). They grow rapidly, and are mainly nocturnal. Their max size depends on the species (be sure to make sure you know what you're getting), but the feather fins usually max out at about 8". Tank mates should be introduced to the tank at the same time as oscars, and be appropriately sized, more on that under Setting Up The Tank. Some fish to avoid: Red Snakeheads; just get too big for most tanks, some other species of snakeheads can be kept with oscars. Red Tailed Catfish; another monster fish that gets too big for 98% of home aquariums, will eat large oscars (and any other fish) when it gets larger. Aggressive cichlids; red devils, umbies, doviis, pike cichlids, etc. Just too nasty (and too big) for oscars to handle. African cichlids; they need very specific (hard) water conditions, and some are too small to be kept with oscars. Piranhas; will often kill oscars, or any other fish in the tank. Convict pairs; breeding convicts, or most other breeding cichlids, get really aggressive and territorial, making the other fishes lives a living hell. Convicts just tend to be a lot easier to breed (it often happens unintentionally).
Water: Oscars originate from the Amazon, which has very soft acidic water. Most oscars seen in the hobby today are bread in captivity, and are more tolerant with water parameters. This does not mean you can keep them in any water. IMO, a low pH of around 5.5-6.2 is best, along with the addition of peat/blackwater extract. If you chose to set your tank up like this, then all other tank mates must be from the same area (eg geophagus, sevrums) or able to tolerate the soft water. Otherwise a pH of around 6.5-7.5 will do, and you will be able o keep central American, Asian or west African fish with your oscars. The temperature should be somewhere around 25-26 degrees Celsius. High nitrates/nitrites/ammonia have bad effects on oscars, including hith (hole in the head/head and lateral line erosion disease), so the tank should be well cycled before the oscars are added, and regular partial water changes should be done.
Setting Up the Tank: Large tanks weigh a lot, and any tank big enough to hold an adult oscar should be placed on a purpose-built stand that can take the weight. As plants and undergravel filters are not usually put in oscar tanks, it is entirely up to your tastes as to how much gravel you add, and what size and colour it is. I usually use a mix of dark brown, gray and that orangeish stuff they sell as "fruit salad" or "autumn harvest", with a size of around 5mm, with some larger (8-10mm). Oscars like to dig, so the gravel should be at least 2" deep. Any large decorations such as rocks, driftwood (a must for pleco's) or flowerpots should be put in before the gravel is added, or placed on the bottom of the tank so any digging wont cause them to fall over, leading to squashed fish, and cracked tanks. Oscars and most other cichlids need rocks, bits of wood, etc to form territories around. They also make places for any nocturnal/shy fish to hide during the day. These should be re-arranged whenever a new fish is added to break up the territories and reduce stress on the new fish. I seriously recommend you make sure all the equipment is in place, and the tank is in its final position before you fill it with water (I sure wish I'd done that with my 150g). Once it is full, leave it running for a few days so it can warm up, and you can sort out any bugs with the filtration. Then its time to cycle it....
Cycling the Tank: There are two ways to cycle a tank, with fish or without fish. I know noting about the fishless cycle, so I'll try to explain the other way. I won't go into too much depth here, because you should have at least one good book that clearly explains it all. Once the tank it warm, and the filters are running you can add a few smaller, hardy fish (giant danios are good). These will produce waste, which in turn will lead to a spike in the ammonia and nitrite levels until the bacteria builds up in the biological filter media. After a few weeks the levels will drop to acceptable levels, then it is safe to star adding other fish, while keeping an eye on the nitrates/nitrites and ammonia.
Maintenance: Daily you should check the equipment is all running properly, and check the temperature. Weekly, you should do a water change of 25-50% depending on the fish load, and clean the filters (this depends on the type of filtration you are using. Do NOT use water from the hot tap to fill up your tank, if you've ever seen inside a hot water cylinder you'll know why. Bi-weekly to monthly (depending on how long your tank has been set up) you should check the nitrates/ammonia/nitrites. Don't do this strait after a water change or the readings will not be accurate.
Feeding: There are a wide variety of foods out there, ranging from pellets, to insects, to live fish. Oscars should be fed a balanced diet, keeping in mind that they are mainly carnivorous. Oscars are pigs and will eat themselves stupid, so don't feed them just because the 'look hungry'. I fast my fish for 1 day a week, I cant remember the exact science behind it, but I've been doing it for ages and they're all healthy. I'm not going to start a big debate on feeder fish, I'll leave it up to the individual to decide weather they feed their fish feeders. I'm not going to talk about diseases (especially hith) for the same reasons.