Instructions
Make sure to obtain an appropriate brand of salt. Salts for use in aquariums should not have any nitrates or phosphates, both of which are ill-suited for tanks. Some good salt brands might include Instant Ocean (IO), IO Reef Crystals, and Coralife.
Fill the tank with a suitable amount of distilled dechlorinated water--you should be able to use a commercial dechlorinator. Do not use tap water.
Place a hydrometer into your water and let it float freely. Make sure that the temperature of the water matches the temperature to which the hydrometer has been calibrated--usually, around 77 degrees Fahrenheit.
Slowly add the salt until the hydrometer's specific gravity reaches a level of 1.015. At this point, remove and clean the hydrometer, and let the salt water sit and mix for 36 hours.
Return the hydrometer to the water. Continue to add salt until the specific gravity reaches the required level for your marine system: 1.019 to 1.022 for fish, 1.023 to 1.025 for reefs and 1.025 to 1.028 for corals and clams.
How to Mix Salt for a Salt Water Aquarium
Aquariums can be beautiful, but preparing them for their future inhabitants takes some special care. In particular, salt water needs to be mixed very precisely, so that the water's 'specific gravity' will be suitable to the type of marine life you hope to house. Luckily, mixing salt and water isn't rocket science--it just requires a little bit of patience and care.