I'm no that familiar with them so hopefully someone else will chime in.
The way they work is by passing water that has been subject to high levels of CO2
to drop the ph
over a volume of calcium carbonate. That water draws the CA
and is then returned to the tank. The rate at which this happens I assume has a maximum. If the water moves too quickly it won't have the chance to draw off the CA
(I assume). So I would imagine the thing to do would be to be sure you get a reactor that is sufficiently sized for your needs. Probably go a little bigger than you think so that as your tank grows (and your corals grow) you still have some room to grow in your reactor.