rusting

Hi Reefers, I have a 146 gal Mixed Reef which is 2 years mature System and i dont have any Algae etc. Alk 8.5 dkh No3 -5ppm Po4 .10 ppm Mg 1350 ppm Tin 25ppm Salt 1.025 All Other parameters Stable. Less Corals Somehow Receding Continuously Specially Torches etc. Changed 50% water Thrice till i...