The general approach is to use the combination of filters that will adequately treat the incoming water, and give you a good service life at a reasonable cost.
water is what you want to drink - not DI
water is nearly pure h20, and it doesn't taste good. You need some minerals in the water to make it taste good.
So - why run your drinking water through DI
resin to strip out all the ions (which costs you money), and then run it through GAC
(which also costs you money) to add ions back in? Simply bypass the DI
entirely and drink the RO
Remember that the RODI
systems we all use for reef tanks like this
evolved from residential drinking water RO
's like this:
So the technology (RO
) to make great drinking water has been around for years and years, and it doesn't involve mixed bed DI
Again - I would really question any water treatment professional who provided the recommendation you rec'd.