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Instead of adding FW for evaporation add SW. This will bring your salinity up. I probably would not add bore than 1/2 a gallon at a time. Just add FW for the rest of it.
You can do it like I accidentally did. Instead of replacing lost/evaporated water with regular RO/DI water, I replaced it with pre-mixed 1.025sg water. That brought up my levels and then, "mysteriously", killed my shrimp.