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RO (reverse osmosis) provides the best way to produce ultra pure water (pure H2O).
It has long been the choice for marine specialists but is now becoming more used in the fresh water hobby.
Most good local fish store provide pure RO water and RO water with salt in (sea water).
Pure R.O. is stripped of everything including KH to give pure H2O (water) it should never be used untreated and on its own; the only exception is to replace evaporated water.
It is easy to overlook the need for electrolytes; mineral cations such as calcium & magnesium and the effect of KH (Carbonate hardness) in the aquatic environment.
Minerals such calcium are essential for osmotic function in fish and many aquarists make the mistake of believing that some fish such as Discus do not require Calcium or minerals when in reality (based on many studies in biochemistry) these mineral cations are essential .