A problem with reef keeping is the terminology. For lack of a better term, the industry uses, “top off” or “topping off”.
Over time, depending on a few factors such as room temp, humidity and system water movement, the freshwater evaporates leaving your system with increasing salinity. Add some freshwater to keep salinity in range.
At this time, we just manually add a gallon or two every day or so. Our 200 gallon system is STILL in progress and the water level in the sump is changing frequently.
We will add an automatic top off system (ATO) at some time in the future. We don’t sweat salinity and don’t try to hold it to any exacting number. 1.023 to 1.025 is our target range.
When we do saltwater change of around 10% each month, the new water is not always a consistent salinity or matching the system currently, but as % of the system, not a problem.