I would plan on getting a bigger tank for an Oscar, 55 galls is the minimum that everybody will recommend but the more space the better. I have mine in a 115 Gall (US) tank with 3 other smaller cichlids (1 blue Acara and 2 firemouths) and I believe this is the maximum number of cichlids I can keep in there (it was that combo or 2 Oscars).
When I originally looked at getting an Oscar, It was for a newly purchased 55g tank, but I am glad I didn't. At a full grown foot long, it wouldn't leave much room!
If you put him into a 30 Gall with any other fish (pleco, etc) there is a chance that with the limited room he will still attack it.
Depending upon the character of your O, he may attack anything in a larger tank anyway, that's where the trial and error come in for you, but you do need to look for at least 55g and more if you can afford it. For my tank bottom I have a gibby plec (currently 4") which will grow to over a foot and 2 pim pictus (fast moving cat fish) which will grow to about 6 inches. I did consider moving some of my hoplosturnum in, particularly when Mark Stone on this site advised me that pictus can sometimes irritate oscars. But so far so good and I think the Hoplos may have been a bit too mellow for when the cichlids grow larger, so I'll wait and see (I can always swap them later). Between the three of them and two pink kissers which I moved in there, there is very little waste food left anymore for me to vac.
Also for filtration, if you can, you should be aiming to have the water in the tank cycled somewhere around 10 times an hour, I have seen it recommended 5 times an hour if using external filters, but, with an Oscar you cannot have too much filtration. I have 3 filters running on my tank, cycling it around 9 times an hour (2 external + 1 internal).
My O is not really bothered about decorations, however, all the other tank mates need areas to get away from him when he gets boisterous, to this end I have plant pots with the bottoms knocked out, 2 large pieces of bogwood and a home made slate cavern (bonded together using aquarium silicon), he does occasionally sit on the highed point of the bogwood when the lights are off at night but that's it.
Hope this helps.
). I was told by my local fish store that he will only grow to the size of the tank so I'm planning on getting a 30gal w/ a 60 gal filter, in about a week. what do i need to do before i put him in the tank, and how should i set up the tank, (i heard flowerpots and plants also flat rocks like slate)
