Water Chemistry

We are having a problem with decreasing salinity. It has only happened over the last couple of weeks. We have an ATO system, and we change water once a week. We use the same salinity measurements every water change. What is causing this? The drop is going from 025 down to 020. I know you have to replace evaporated water with RO, but the salt should stay because it doesn't evaporate. Any suggestions?