Di system add on placement?
 

I recently bought a bulk reef add on di system for my cullingan water tower ro unit. The di unit was a line in and a line out. My question is were do I want to add this system. I have two options

1. After the holding tank so the water only goes through the di unit the one way but the water will flow through it at a quick rate

2.inbetween the ro unit and the holding tank but then when I open the tap to draw water it will be running backwards into the filter

Thanks in advance

Are you speaking about a tap for drinking water at the sink?
If so, Tee off that line to the DI
Flow chart:
RO-holding tank- tee, one to sink, other to DI then to pail, brute can, what ever you use for aquarium.