Here is some information on RO and RO/DI water:
Reverse osmosis (RO) water: This is an excellent water source for use in a SWA. It has no ammonia, nitrates, chlorines or chloramines, and it generally has a very low phosphate count if any. It can be purchased premade from many places including the LFS, some grocery stores (the bulk water stands by culligan are RO filters), and home RO units can be purchased for relatively little cost over the long term.
De-Ionized water: This is another filtration step in the process or reverse osmosis. The water passes over a special resin that removes virtually all impurities of the water. This is the BEST water source available to the home hobbiest. RO/DI filters can also be purchased for a relatively little amount of money when considering the long term.