whenever you do a
water change, just rinse the filter media in with the used water (whatever water you just removed from the tank), and just get most of the gunk off of it. you shouldn't ever replace your filter media until it is falling apart, and even then you want to run the old media and the new media together in the fitler so that enough bacteria builds up on the new filter.
also, i may have mentioned this before but your oscar is probably the reason that your water is so dirty. they grow much too large for a 20 gallon tank, the minimum tank size that i'd recommend for just one oscar is 55 gallons.