Moved to hardware/equipment forum.
You need a
ro/
di unit if you have any trace of
no3/
po4 in your tap water. If not it's not needed but still helpful in providing the purest water possible for your tank.
RO only water can still have
no3/
po4 but
ro/
di filters out all harmful elements in tap.
DI alone isn't recommended either since the resins are used up quicker.
EBAY has a ton to choose from at reasonable prices.
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