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
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."