Use Ro/Do and a hand heald TDS meter and change the DI resin when it gets above 5-6, is that OK? I just resived my new Milwaukee Refractometer, tired of the hand helds door breaking all the time and having to calibrate every time and getting several diff. readings. Having a Red Algy/Cyno out breake and read where a reefer just blacked the tank out for 3 days and all cyno gone and his crushed coral was white once again he stated, thoughts??