Di system add on placement?

[copenhagen_cowboy@live.co]

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