Sorry to hear about the problems Estaban, You need to leave the bottle of calibration fluid in your sump and calibrate with test fluid the same temp as your water you are testing. Salinity changes with temp on a refractometer, so if you calibrate and measure your water at 77 degrees, you will get a different reading when your tank drops to 74. So if your bottle of calibration fluid is at 65 degrees in garage and you calibrate then test your 77 degree water it will be way off in accuracy. I like the Apex as well and keep mine at 35ppt with an email if it goes up or down 1ppt.