You replace evaporated water with fresh/pure water. because when the water evaporates, it leaves everything else behind.
You will have to add top up water often, as the amount that evaporates is quite a lot more than a fresh water tank. calculating how much water to replace is probably bloody difficult, it's better to add a mark on the outside of the tank to let you know where the water height should be for when you replace the evaporated water.
Also, NSW is higher in salinity ~40ppt, so you will have to dilute it with RO/DI water to get to ~35ppt. Used a refractometer to measure this.