I'd strongly advise against using tapwater. There is more in your tapwater besides just the chlorine that will turn into algae fertilizer. The decholrinator will remove the chlorine and maybe some metals, but nothing else. Letting it sit for a few days as you described will do nothing additional for the water. You can use water from one of those machines at a grocery store (which is generally RODI
water) as long as you test for nitrate and phosphate in it to make sure it is actually pure. Using a TDS
meter would work as well. Sometimes the filters wear out in those machines and don't get changed as often as they should. Many LFS
that specialize in saltwater sell RODI
water or premixed saltwater as well. I'd still test that on occasion as well.
If you start with freshwater for the water change (as opposed to pre-mixed), you do want to add the salt, check specific gravity with a refractometer (more accurate) or hydrometer to adjust to 1.023 - 1.026. you want to keep it close to what you ahve in your display. Do allow time for the salt to dissolve completely and using a pump to circulate the water inthe bucket is a good idea to speed the process up. Do use a heater to get temperature close to what is in the display as well.