Berkey is not R.O. but works well.
For R.O. just get an ebay seller.
The options are:
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"how many stages" 5 is enough for 99%
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UV sterilizer (if water is chlorine added, probably dont need)
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Tank size (bigger better)
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Permeate pump (filters better and less waste, more pressure in tank) - recommend it if can afford.
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booster pump (for low supply water pressure). Membranes work better if high pressure.
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DE-Ionizing filter (not even suggested for drinking, but your choice)- for fish tank use mostly.