straight from LEE....
1. Make sure you're using proper source water free of contaminants (including but not limited to: dissolved organics, ammonia, nitrites, phosphates, nitrates, silica compounds, smell, and poisons). If there is any doubt as to the quality of the source water, test it. Artificial salt manufacturers who recommend using tap water as the source water are wrong. The variations on tap water around the world make it such a wide range of ingredients in tap water, that tap water isn't consistently reliable enough for source water for the marine aquarium.
2. Mix the source water, preferably using a submersible (inside aquarium) pump (not an aerator). The water should move up and down in the mixing container, not around in a circle. [Adding air to the water is not desirable, IMO because it encourages carbonate precipitation as the water is encouraged to keep taking on air.]
3. Add the artificial salt to the water, in the quantity required to get close to the marine system's (or QT water's) specific gravity.
4. Mix the salt according to the salt manufacturer's directions (as to how long to mix). (NOTE: The salt manufacturer should know how best to mix their salt into water, however studies have shown that the most stabilized water is achieved after about a week of mixing. This extended time is connected to gas exchange and the chemical reactions going on in the water between the various salts and the gases in the surrounding air.) In no case would I recommend mixing for less than 48 hours. It has been discovered that bad salt mixes will usually 'show themselves out' within the first 48 hours of mixing. If the mixed salt is cloudy or undissolved in 48 hours, there is something wrong. (NOTE: Some unusual/less common artificial salt manufacturers may depend upon mixing to chemically react their chosen ingredients. Such manufacturers can recommend up to and including 2 weeks of mixing!).
5. After mixing is complete, check the specific gravity of the prepared water. [NOTE: If the salt mix is not properly balanced to provide the proper calcium, alkalinity, and magnesium, now is an opportunity to bring those chemistries into the desired range with the chemistries in the marine system. This is optional.] The specific gravity of the new water should match as closely as you can with the specific gravity of the water it will replace. Usually, matching to within 0.001 sp. gr. unit is acceptable. Adjust the new water by adding a little more salt, or adding some source water. If a lot of salt had to be added (more than 2% of what you've already added) then go back to 4.
6. When the specific gravity of the new water matches the marine system water specific gravity, measure the pH and temperature of the marine system and the new water. Adjust pH and temperature of the new water to that of the marine system water. (NOTE: This is one of the places of the biggest common error -- the pH adjustment. The pH of the new and old water must be extremely close -- to within 0.05 pH units, if possible). The temperature of the new water should be no lower, and can be up to 1.5F higher than the marine system water.
7. Remove the water from the marine system. You can take advantage of siphoning in order to clean out detritus/debris around in the display tank, sump, substrate, and/or refugium.
8. Add the new water to the marine system.
After the above, it is important to replace evaporated water with distilled water (if you have a small aquarium), or RO/DI or deionized water for larger systems. Maintain a constant specific gravity of the marine system, a constant pH in the proper zone, and a constant temperature.