1. You can not use common table salt! Use a good sea salt mix. What to Look For and Compare When Choosing a Mix to Buy will help you choose the best sea salt mix for your aquarium.
2. If you are starting a new (unoccupied) tank, fill it with either R/O or filtered tap water. If you are adding water to your tank for a water change, you can use a large clean container (5 gallon plastic bucket or larger container like a garbage can) to mix the sea salts.
3. Add the sea salt mix to the water (start off with less than you think you will need. It is easier to add salt than it is to take water out, then add more freshwater)). The sea salts will dissolve faster if the water is agitated. You can stir the water with your hand (for a small container) or with a powerhead (a lot easier and faster). Mix the water until it is clear.
4. Test the salinity (SG or Specific Gravity) of the water with a hydrometer or refractometer. The salinity of the water should be the same as in your tank if you are replacing water in your tank. For reef tanks with corals, the salinity should be between and 1.023 and 1.025. A fish only tank should be between 1.019 and 1.023.
5. Keep adding the sea salt mix and stirring until the SG is where you want it and the water is clear.
6. If you are adding water to a running tank, the temoeratures should be within a degree or two. Dropping an aquarium heater into the water container will help adjust the temperature to where you want it.
7. Add the new water to your tank. If you are adding the new water into a tank that is already set up, putting something on your substrate (a clean dinner plate works well) will help keep your substrate in place.
8. Rinse your container, powerhead and hydrometer or refractometer with fresh water for storage.
Not too bad, was it?