4 oscars in a 55g? That tank isnt big enough for 1 oscar.
0.25ppm ammonia is a sign that your tank isnt cycled. The only way to take it down is through getting your tank cycled or water changes. 4 oscars may simply be too much bioload for your system and you will always be fighting against water parameters and the 55g doesn't give enough space for them to live comfortably.
What are your plans for dealing with the overstocking? Returning fish or getting a bigger tank?