Nice build.
Re the issue of top-off from RODI vs reservoir addressed above, consider the SpectraPure UHE unit. When it is done filling your reservoir (e.g., a 50 gallon bin), it will automatically switch things around to draw water back from the reservoir to flush the membranes, and then when the unit shuts down it will leave the membranes soaking in the RODI'ed water.
The unit is controlled by two float switches. One low, for when reservoir runs low and needs to be refilled; one high, to turn off the unit when the reservoir is filled.
I have installed the unit and have been using it for a few months to supply water as needed for a 12 or 14 g desktop tank (e.g., manual topoff, water changes), and I have been very impressed and pleased with the unit. I am in the process of setting up a 250, which this will supply. When the tank arrives and I get to that point, I will add two extra float switches to the reservoir, to control operation of the unit. The additional low switch will sit below the low switch per the original UHE installation; if water drops that low and triggers the switch, Apex will send signal that something is wrong. The additional high switch will set above the high switch per the original UHE installation; if water rises above that switch, Apex will kill power to the UHE unit and send an alert.
(Topoff will be handled by Genesis Systems auto topoff and water change unit, which comes with float switches to control. Apex water detector on floor by sump will send alert if water overflows sump and will kill power to the Genesis system.)
FWIW, and HTH. Now back to work for me!