There are two ways to make "pure water"
If you water supply has a low TDI and after going through a reverse osmosis system you could have 000 water even if it comes through 000-12max you could use this every time WITHOUT any Di vessels.
If you do not have a reverse osmosis system you could make 000 water through Di as well.
To do it through di you would need one 25ltr vessel for filling into your tank or water carriers ,bottles etc and then another 25 di vessel from your water containers through the pole.
The reason for the di filler is because you wouldnt be able to remove as much minerals this way than a reverse osmosis.
Also using resin only depending on how much water you use then the cost varies.
When i first started i had one di filler and was running two di for two poles.
We got up to using 700ltr of water each day but three vessels needing filling with new resin every 2-3 weeks costing £300.oo plus so reverse osmosis is the more economically way to do it.