You need to do 3 things,
1. Perform weekly water changes at about 10 to 20 % with the aid of a gravel vac.
2. Use an algy mit to clean the interior of the tank.Glass, heater, filter tubes etc.My Oscar nuzzles on my arm as I do this !
3. You need to get a bigger tank pretty soon around 75 gallon should be o.k.
A friend for your Oscar my be an idea too, once you get the bigger tank.
Mister Malcontent