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Maintaining your salinity can be done a variety of ways. A simple way to do it is to use an automatic top off system that will replenish the evaporated water for you. Or, you can just add fresh water each day to replace the evaporated water. As water evaporates, salt doesn't so your salinity goes up until you replenish the water. If you're running a sump, it's really easy. Put a piece of tape on the outside wall of the sump tank to mark the water level, then just refill to that point each day.
It is possible to be successful using dechlorinated tap water, but the risk of algae and other issues is much higher. It's highly recommended to use RO
water if possible.