Preparing tap water for use in a saltwater fish tank can be one of the most time consuming and mundane tasks of fish keeping, but it is also one of the most vital to tank maintenance. Plain tap water is far from suitable for use in a delicate saltwater tank, so tap water must be treated, mixed and preferably aged before adding to your tank. Having a large plastic dustbin on hand can make the mixing process much simpler.
Fill a large bucket, tub, or clean plastic dustbin with water from the hose or tap. Alternatively, if you are filling up a new, empty tank with water for the first time, you can mix the water directly in the tank.
Add an aquarium water treatment solution to the water to remove chlorine and chloramines. Be sure to read and follow all packet directions carefully, as water treatment products vary from manufacturer to manufacturer.
Select a commercial salt mix for marine aquariums, and add the appropriate amount of the mixture according to the instructions on the packet. The salt should be added to lukewarm or room temperature water for best absorption, and mixed well.
Use an aquarium hydrometer or refractometer (available at most aquarium supply shops or from online suppliers) to test the specific gravity of the water, aiming for a reading between 1.021 and 1.024.
Place an aquarium power head into the bucket or container of water, and turn it on.
Allow the power head to run 24 hours before adding the water into the tank.
Mix small 4.5 to 9 litre (5 to 10 gallon) batches of treated, salted water the day before planned aquarium maintenance, to be sure that all future water introduced is safe for the established tank.
Salt mixes are available from a variety of manufactures, and each brand varies slightly. While just about any mix from a reliable brand name will work for a marine fish tank, reef tanks may be more sensitive and require a reef-specific salt mix.