Jef, i'd start w/your RO unit, as you said last week your TDS was like 15 or something, if i remember correctly. i run 2 of the two little fishes ones. one has rowaphos, and the other has carbon. i run two, because they need different flow rates between the two, and the carbon one gets changed more often. both are fed off my return pump.
i do have an old style reactor you can have. it's the old PITA type, with the 16 thumb screws :/ lemme know if you are interested. i can leave it at the front door for u.