One thing I notice when I do my weekly manual topup of my reservoir...there is TDS creep every single time. That is weekly. I can also turn the ro/di off for 10 minutes and still get creep.
So what I do is bypass the DI resin for a minute or two until the TDS after the membrane is 3 or less. then I run it through the resin and into wherever I want good water. This saves your resin from a constant butt kicking.
In my opinion doing manual fills in this way will allow your media to go much further. Not that you asked for my opinion but...
Fish room's looking food btw!