Conditioning tap water is needed to be done if you are to use your own water to make saltwater for your aquarium.
Most people use water straight from the tap. Using bottled or filtered water costs too much and not feasible unless your tank is ten gallons.
The water in your tap is not pure water. Most local water municipality chemically treats the water for health reasons.
The typical treatment includes chlorine or chloramines, which can be toxic at level much higher than found in natural seawater.
Tap water also contains nitrates, sulphates, phosphates and heavy metals. These compounds are harmful to fish and invertebrates.
You have to condition your tap water to remove these chemicals. The easiest way to do this is to let the water sit for 48 hours and let the chemicals diminish. But this wont to much to remove high levels of heavy metals or other compounds.
The more efficient way in conditioning your tap water is to purchase a chemical water conditioners and dechlorinator. Local dealers offer variety of water conditioners. They would also probably know which is the best one for the water in your area.
These conditioners can be added to your water to remove chlorine, chloramines and heavy metals.
There are also water-purification units available to remove high levels of phosphates and nitrates from your tap water.
These reverse osmosis and deionization units attach to your faucets. They can remove a number of substances like nitrates, phosphates, toxins, heavy metals and chlorine. This will of course depend on the unit that you intend to buy.
These units can be expensive! But if your water supply is known to have high levels of these toxins, you will probably need to get one.