Hi, i have the same problem in my tank for about 6 months and its huge like everywhere i removed the sand i even change my live rock but nothing... Now i have been followed this tread and i believe that make sense what Eastone said about the nitrate because i have the same readings 0.00 nitrate(using redsea test) , however i will try now the red fied ratio calculator to try bring nitrate and phosphate on balance to do that i will also add potassium nitrate .