Let me say I've used both of these, and it wasn't until this week that I noticed some discrepancy. Over the past week or so, I've been wondering why my parameters have been elevated. I couldn't figure it out. Alkalinity was at around 9-ish, Calcium just below 500, Magnesium at 1450/1500..The salt mix of choice has been Fritz RPM. This won't become a salt mix debate. Since I've rebooted my tank, I've been making water at 35 PPT, which is widely known as NSW (1.0264) or whatever. Cool right? Fritz RPM at 35 PPT mixes up at roughly 9.0/475/1500. They say it should be 8.5/430-450/1300-1400. Now, I just made 5 gallons of new water, mixed to 1.025, which equates to 33 PPT. The tests were 8.6/440/1400. Basically identical to what the manufacturer has said. I have used Red Sea (Blue Bucket) previously and at 35PPT, the Alk is 9.0/500/1500. At 1.026, the results are 8.4/430/1400 (if I recall), but the PPT for this test was 34. Regular Instant Ocean: at 35 PPT, Alk was 11.2/465/1480. At 1.026 (34 PPT), 10.0/440/1400, or there abouts. Equipment used: Milwaukee Digital Refractometer: Rinsed with RODI, Calibrated with 1.025 solution, rinsed with RODI, and tested newly mixed water. Hanna Alkalinity dKh checker Red Sea Professional Calcium/Magnesium test With that being said, I am wondering if I should start mixing to the Specific Gravity instead of the PPT scale? I'm very confused as to why 35PPT is giving higher numbers, even though it says 1.026, but if I test to 1.026, and change the scale to PPT, it shows 34 (PSU 33 I think).