Do you ever wonder how people's aquarium or reef systems stay so clean or at least much cleaner than your own? The main reason is because most aquarium hobbyists use some type of reverse osmosis system to obtain such a clean purified water. There are so many benefits to using reverse osmosis (ro) systems for your aquarium including less maintenance on cleaning tanks, less algae growth, and it's easier to pH balance the water.
Tap water more than often contains impurities that cause problems when it is added to the aquarium. Most tap water contains phosphates, nitrates, chlorine and heavy metals. Aquariums are sensitive to phosphates and nitrates which encourage algae growth. Heavy metals like copper are typically found in your tap water and is very toxic to invertebrates. A reverse osmosis system will easily remove these impurities to a very low level which is why they are so popular with reef/aquarium hobbyists.
Typically most waters, especially well water, has a high hardness level which makes it very difficult to pH calibrate. Hardness is defined by having calcium and magnesium present in the water. pH conditioners work best when the water source has very little hardness. The more impurities in the water, the more pH conditioners you must use in order to balance it correctly. However by using a ro system, it will remove the hardness so that pH calibration will be less costly and much easier.
For ultra pure water, you should combine any standard reverse osmosis water with a deionization (mixed bed di) filter. A mixed bed DI filter will remove all the left over contaminants that a reverse osmosis filter cannot remove which will leave virtually nothing in the water. A combination of reverse osmosis and DI filters will bring your water to a 99.9% purity.