Salinity spike??

This is most likely your issue. Salt mix and water should mix, with the flow of a powerhead or pump, for at least around 12-24 hours before adding to the tank. If not, the mix can seperate, and the salt will become trapped in the sand and leach back out into the water, raising your salinity levels
If you rotate out for fresh, try cleaning the sand with a vacuum siphon when you do it.

 

You may be heading for trouble real fast! If you can't get an accurate reading you may be lowering the salinity to a border line reading of .017-.019. Not great for coral. Either take some water to LFS and calibrate the hydrometer or get a refractometer.

 

Hyrometers are inaccurate. MIne was always between 1.024- 1.026. I bought a refractometer (on Ebay it was cheap $20 plus shipping) I got it home calibrated it and my levels were 1.030. I changed a gallon of fresh water a day till I got it down to 1.025. It sounds to me like your If your checking that often and only adding fresh water and still getting inconsistent readings. Toss the hydrometer it will just make you crazy if everyone looks good I would slowly bring your levels down.