How to calibrate refractometer for Hyposalinity

[Justin_Reef]

I have a tough time figuring out if my tank is 1.008 or 1.005 (yikes!!). It seems to depend on how I calibrate my refractometer. I have always used RO/DI but I recently bought some calibration solution which is 35ppt or 1.026. When I use RO to calibrate, my tank is currently 1.008 but if I use the calibration solution, it is 1.005. The second is a little hard to believe as the fish look happy and healthy and as far as I know, that is dangerously low.

Any ideas?

 

[Jay Hemdal Verified Community Expert]

I’ve had trouble with seawater calibration solutions, two different ones, never seemed to work for me, I attributed it to user error (grin). Because hyposalinity is so critical, I use a German glass lab hydrometer as a standard. You might check with Randy over at the reef chemistry forum...

Jay