I have seen systems like yours with denitrate filters on them, and the nitrates still can't be kept in check. The only way I see to get those nitrates down is to do massive water changes. Even if you could keep enough macro algae to keep the nitrates down, you'll never be able to keep up with the other water parameters. New salt will add all the trace elements, calcium, iron and magnesium that you need. I think that by not doing water changes you may be making more work for yourself. PS I do 30-50% water changes on most of my reef tanks 1-4 times a month, and I really don't have to add anything other then some Iodine.