From The Aquarium Wiki
Distilled water also known as de-ionized (deionised) or RO water is water that has been cleaned and purified to remove at least 99.5% of all non-water elements and ions.
The wording may vary but the end product is the same - pure water.
- Distilled = Water is boiled and the steam collected.
- Deionised (DI) = Water is passed through resins to remove ions and impurities.
- RO water = Water that is passed through a membrane to remove impurities.
- Pure water will have a ph of 7.
- Pure water if left exposed to the air will absorb CO2 and shift slightly towards the acid.
- Pure water if supplied sealed may be low in saturated oxygen.
- Pure water by its nature contains no essential minerals essential for the growth of organic life. So its' use in the aquarium is limited.
- To dilute sources of water that may be too hard (GH). For example often used with Discus fish and Marine tanks.
- To prepare solutions of chemical powders for aquarium treatment. See PMDD.
- To hatch out Triops creatures that only hatch out in low GH water.