You are always going to have a risk of aggression with housing 2 oscars in the same tank. Even if that tank is big enough to house thier bioload. untill you can get your oscars into a setup of 110 gallons at least for the two of them, you will continue to have health issues with them. And just to mention removing the sand to clean it is not a good idea, if your sand was cleaned b4 you added it to the tank there should be no need, and is actually bad to clean it. Simply stirring the sand with every water change is enough to clean it.