turn white as in bleachin or rtn? wat abt other parameters of ur water condition like dkh, ph, no3, po4, mg......etc. Measurin juz the Ca level is nt enough to justify wats wrong with ur sps.
Light level r good? how good is good bro? Sps can still die on u even when place under 400W of MH with good Ca level but the rest of the parameters screw up, wrong placement of sps.....etc.
Perform a detail check on ur water parameters and in the meanwhile determine is tat bottom "turn white" sps bleachin or rtn. U can try usin lugols solution for treatment. After dippin it in lugol solution, if "whitenin" spread. Then advisable to discard ur sps.