Stablity is the key, in your opening statement you said that you changed 24 out of 75 gallons every two weeks. that alot of change and if thats new saltwater that hasn't sat for 24 hours or if it doesn't match temp wise, its going to upset the balance in your tank.Also, try to figure out what is causing the nitrate and phospate readings and reduce them.your other parms are ok. good luck