Am I using my refractometer wrong?

[kelper]

Hi, I bought some saltwater from my LFS, and I measured the salinity to be 1.025 this morning. I used it to calibrate my refractometer. I checked the same saltwater jug at night, and the salinity has gone down to 1.021. How can this happen? Shouldn't the jug of saltwater always have the same salinity value? I didn't add any chemicals or dose the saltwater at all. It's still the same old jug of saltwater. I wiped down my refractometer between uses.

[sadie]

I just found out that temp has something to do with salinity.  Your refractometer might not take that into account? (My old one did not, new one does)  Try getting the the jug of water the same temp as your tank, then test both.

[Tired]

Float a sealed container of the water in your tank to get the temps the same (or float a bag like there's a fish in it), and that'll do it.