I had my Oscar for 10 years before he died of hole in the head! It was really sad for me and I even went out of my way to see that he had a proper burial. He ate anything and everything, but if your Oscar has the proper personality and is friendly it will protect the tankmates it has come to know for many years. There was a small Catfish in my tank that did not grow over 5 cm, but my Oscar grew to be about a 30 cm long. When the Oscar was initially put into the tank, he was a bit smaller than the Catfish. They grew to be (in my eyes) what seemed like friends. Every time I would put in a new fish in the tank, the Oscar would test it. If the new fish attacked the Catfish the Oscar would eat it or attack it. If it didn't it was left alone. Very strange but it shows you how wonderful Oscars truly are. I now have 2 new Oscars and they are great. I hope they have similar personalities to the old one, nevertheless they are great to own.
I have two Tiger Oscars, one black and one albino. After reading all the comments, I had to tell my story. I have a 200 L tank, which I know is too small, and can't stop watching it. My black Oscar is the meanest thing anyone has ever seen. I don't know how he got that way. When I first bought him I had three other fish. All of them twice his size. Well, after a few weeks, he managed to eat all their fins off. So, I then bought a Jack Dempsey, because I was told they were tough fish. Even though it was the same size as the Oscar, it lived for two days. So I did the only thing I could and bought another Oscar. As soon as I put the albino in the tank, I could hear wedding bells. They are constantly touching and rubbing. They even share food, it's very weird. They both love Goldfish, but they go crazy when they see a tuna can! It's like they can read. Someday I want to try other fish, but only when I can afford a new tank. Charlie (Charles Manson) would eat anything else I put in there.
I have 5 Oscars, 3 Tiger, 1 Ruby Red, and 1 Black. All three Tigers are adults and are between 25 and 35 cm in length. The shortest of the three, I bought when he was teeny, and now he's huge in less than a year. The other two I bought from a previous owner who was not capable of caring for them any longer. I would love to breed them, but am having extreme complications with the sexing principles I've read and seen posted. I love my Oscars and they are the best fish by far to have in a tank. Very very intelligent creatures.
These undemanding fish are often the aquarists's first encounter with Cichilds. The young fish are very attractive, but unfortunately grow rapidly to a large size. They are undemanding as far as water is concerned, tolerating soft, hard, acid or alkaline, just as long as extremes are avoided.
I have two Red Oscars. I don't know how old they are but one is about 22 cm and the other 16 cm. Once I fed them 24 Goldfish and they ate them within six hours. Sometimes they will only eat five each day. The only thing they eat is something that moves. I tried green beans (on someone's advise) and as the beans floated they never gave them a look. They will eat shrimp pellets sometimes but usually spit them back out. They share their tank with a striped catfish.
Right now I've got 5 Oscars (4 Tigers and one Red) in a 450 liter tank. I do agree with most of the owner comments, Oscars are just like having loyal dogs, you'll never find a friendlier fish. I've owned Oscars for over 10 years, and I've only had a "cannibalism" problem with two of them. And they, of course, were bought at a larger size, and had probably been accustomed to eating live food. My current Oscars were raised with Goldfish and wouldn't think of taking a bite out of one of them, even when I forget to feed them once in a blue moon. As for the ammonia, I can't stress how important the proper filters and filter media are. I've got a Fluval 4 on each end of my 450 and a ton of carbon/charcoal/ water treatments. Keep your eyes peeled. Oscars are tough fish, they can live through tons of abuse. So if they don't rush to greet you when you come over to the tank, or if their markings look faded, or if they don't have interest in dinner, something is terribly wrong, start testing, changing the water and cleaning filters!