ammonia reducing filter media?

RO water may contain small amounts of ammonia if your water utility uses chloramines as a residual disinfectant. DI resin removes the final traces of ammonia.
Even so it will be in the 1 to 2 mg/L range at most and probably significantly lower as RO is about 90% efficient at removing all forms of ammonia, nitrites and nitrates. An established tank with live rock should have no trouble with smal levels like that.