They make a murlock ball valve that you can put inline of your product water. You can then manually split the line and send the secondary line to your DI cannisters (I would definitely use 2 in your case) and then fill a reservoir container for your tank. You don't want to drink DI water anyway!
That is likely your biggest problem, since it is making you drive your nutrients down. The thing you haven't considered is what all the other stuff is that is making up that 30TDS, probably a grab bag of stuff, maybe heavy metals etc. best to fix this first! I'd also lay of the NOPOx for awhile until your nutrients start to rise then you can adjust the dosage to compensate. I think you have likely built up a huge surplus of carbon in the system, which is compounding all your problems with your make up water.