washing soda

50 gal tank is 6 months old, SPS dominated. Everything was going great then most of the SPS turned very pale 4 days ago and I've lost 10 of them since then. sg - 1.025 cal - 450 mag - 1350 alk - 10.5 dkh pH - 8.0 nitrate - 0 with API test kit, 10 ppm with Elos (consistently) phosphate - 0.05...