nitrate issue

So i have had this 60litrs nano tank for about 6months now. I have two birds nest coral frag, one funghia, few zoas, ome chalice and one hammer. Fishes only 3. Initially my params were:- Dkh-8-9 Calcium-420 Nitrate- 0 (negligible) Salinity - 30ppm All corals were growing healthy and hammer...