My tank had the same results in about the same time. IMO
it sounds like your cycle is nearing the end. Did you use uncured lr
or shrimp to start the cycle? What were your readings before?
65 gal reef
2 PC's 192W each , 20 gal sump/fuge
4 false clowns, 2 firefish, cardinal, midas blennie, toadstool, zoos, shrooms, trumpet coral, torch, pagoda, frogspawn, poylps, red open brain, devils hand, hermits, snails, 3 cleaner shrimp, coral banded and a emerald crab.