If you want to avoid displacing gravel and what's in it, put a small plate underneath the stream of new water to diffuse it as you add water slowly. If you want to avoid having nasty water after a water change altogther, cut back on feeding and increase gravel siphoning. If there's filth in the gravel, it's creating Nitrate, (hopefully only Nitrate -- I don't know if this tank is actually cycled), and therefore, even when your water looks clean, because the muck has settled into the gravel, it's still high in Nitrate, which will lead to illness, such as HITH, which is common in Oscars. It's best to remove it, rather than work around it, for the health of your fish.