This unit sounds like a regular drinking water unit. I say that because there is a carbon stage after the DI resin. For aquarium water you don't need the last carbon stage, this is used to help with the taste of RO/DI water for drinking.
Are you getting a dual TDS meter? Dual meter are installed with 1 input before DI resin and 1 input after DI resin. This will tell you the resins performance.
"If it was me" I would put a T before the DI resin. From the off shoot of the T I would put a valve then hook up the carbon stage to that for filtered RO drinking water. "Just me not saying you really need to do it."