You're actually supposed to use calibration fluid instead of distilled or RO/DI water. You should use a fluid that is close to the salinity you are measuring (as with any calibration fluid). I use Pinpoint Salinity which is 35 ppt (~1.026). Works very well.

Wait I'm confused, I thought you used distilled water as a 1.000 gradient (Which set the scale) and then that was the comparison to your salt water. Ahah watch my salinity is actually at 1.015 oh dear crash here I come.

Edit* Just checked my instruction pamphlet (On a Vertex Refractometer) and it says to use distilled water, what type of meter do you use myka?
Levi

mine uses distilled water also and I believe most do I have checked out quite a few Refractometers. Mine came with distilled water measures to 1.000 gradient also. I have checked freskly mixed satlwater and some that has been mixed for a week or 2 wating to be used.

Bill

The problem I encountered was that the paper instructions said to use distilled water for calibration; yet the calibration fluid had different directions on the bottle (which i didn't fully read until it was too late).
Distilled water = calibrate to 1.000
Calibration fluid = calibrate to 35 ppt (or 1.026)

So I used the fluid and calibrated it to 1.000 instead of 1.026 (35 ppt); and threw my readings out of whack.

I was worried about the fluid because when it arrived (via UPS) it was frozen; therefore i wasn't sure if it could be trusted for accuracy.

So far the corals seem okay. I just my readings again and it has gone from 1.035 to 1.033 overnight; so slowly but surely its going back down (due to raising my ATO float switch). Another water change later today (with approx. 1.020 water) should help.