For every gallon of water an RO filter pumps it dumps 2-5 gallons of "waste" water down the drain. The amount of water that goes to waste depends on how efficient your RO filter is and there for cost of the unit. Looking them up (been years since Ive used RO) you can get as low as 1:1 now for the cheaper units which is decent. It still means for every 40 gals of water out of the RO filter at min you will dump 40 gals down the drain.
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It's actually called a Tall Boy not Hiboy (unit from brand Hydrologic). It is a filter to remove the chlorine while leaving the minerals. It's good if you have decent water to begin with and want to use bennies or go organic. Something like your case at 590 it wouldn't be.
As far as your PPM and PH going wacky are you in DWC? If so yes that matters.