So are you adding salt to the system after topping off the water, in order to raise the pH?
If you are, you need to stop. Water evaporates, salt does not. If you add salt to raise pH, you raise salinity. As the water evaporates, salinity raises more until you top off.
You may get some salt creep, but it's not enough to drop the salinity in the short term (several months). Use the RO
water to replace the evaporated water. If your pH is dropping below 7.6 then you have something else going on that needs to be addressed, such as oxygen exchange or light cycle. I maintain several tanks, each one using the same RO
and salt mix, and each one maintains a different pH. If you in the 7.6 to 8.4 range you're generally fine, as long as it doesn't swing all over the place during a normal 24 hour period.