From FrugalReef

Reverse Osmosis is a method of water treatment that removes most organic and inorganic particles and bacteria. Water is forced with pressure through a filter membrane that only the water can pass through. The water passes through the membrane and is collected on the other side, the contaminates do not.

Reverse osmosis is the preferred method for obtaining aquarium water and water treated this way is usually pretty easy to find. Walmart sells it cheap, most pet stores sell it pricey, and a RO unit can be installed in the home for a couple hundred dollars.