I would do DAILY water changes treating the tap water with prime ...change at least 50% a day until your ammonia, nitrites are 0 and nitrates under 20..you are going thru a cycle...

the tank is overstocked as one of mine is, so you may have to do daily changes forever...goldfish are big poo'rs and need at least a 30 gallon alone..as well as oscars are big poo'rs as well..but im sure you know all this....so for now, its a must changing the water daily.....otherwise slowly the fish will die off..sorry goodluck!