A Reverse Osmosis De-Ionization (RODI
) unit runs water through a set of filters creating nearly pure water. Many freshwater keepers and almost all saltwater keepers use these systems. Once you have pure water (no chemicals or minerals) you may then add minerals or salt to it bringing the water quality up to a level your fish are comfortable with. They can be expensive and you will need to be prepared to store water as it's being made. There are upsides and downsides to RODI
water. Worth checking out though to see if it's for you.