Refractometer/Hydrometer/GHL

[sphelps]

All the calibration fluids including the fresh water should the same temp as the tank. The profilux has a density offset that also needs to be set after calibration. If it's a good quality refractometer and calibrated properly I would use that as the guideline and set the profilux offset to match. You can try RO or distilled water with the refractometer but it shouldn't make a difference.