I do a 25 - 30 % water change once a month.
Then I rinse all my fiter pads in the dirty water.
When I'm not doinga water change I just use tap water, I know someone will not to do it but I wring out the filter pad very well.
I mix my salt water ahead of time ( with RO water ) in a 30 gal bucket/ garbage can form HD, put a lid on it and some times I will put My heater in it, sometimes the water is a bit chilly. Usually I alloy it to come to room temp for 24 hrs. Then I add it to the tank. You might want to get a small pump to transfer the new water to the tank.