RO filters are based on percent removal which typically ranges from 75-90%. So if they are starting with water that has GH of 18, then the meaurement you got would be 5. So if you're running well water through it, which typically has a lot of minerals, then RO will leave a lot in. If the water source is surface water from snowmelt or rain, the water will have much less minerals and the RO unit will produce cleaner water.
Because RO has the potential to leave a lot of stuff in the water, DI cartridges are used which will bring that 75-90% removal up to 99.99%. So if you can find RO/DI water, it will be cleaner than just RO water.
Also, the pH of pure water will typically be around 6.0 because atmospheric Carbon dioxide dissolves into the water and turns it slightly acidic.
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