Lowering the SG
/Salinity is not as stressful on the fish as rasing it. You can do lower salinity partial water changes as people suggested. Or what I would do is is replace water from you tank with Temp and PH adjusted RODI
water and lower the SG
by .001 every day until you are where you want to be. So day one you would go from 1.030 to 1.029, day-2 you would go down to 1.028 etc etc. Read up on hyposalinity treatment for Crypt/Ich, it should give you an idea of what percentage of water you need to replace with rodi
water to lower your size tank down by .001 per day.
Like mr_x said, you should recalibrate your refractometer often. I recalibrate mine before every partial water change using a refractometer calibration solution. The solution is more accurate than using rodi
or distilled water because it is closer to the range our tank is, like 1.025 or 35ppt