Ok so should I add a secondary chamber along with 818?
It's all about how you tune the reactor, I always recommend people to go bigger than smaller so media replacement is less of a hassle, like mentioned above.
A calcium reactor's turn over or "turnover height" has a lot to do with how efficient they are, recirc pump gph can impact this as well obviously the faster the acidic water passes through the media, the more saturated it can become because of increased ( height)turnover/ contact time before exiting the reactor, more media =more surface area, bigger reactor chamber= more time.
That's always been my understanding.
However it all comes down to tuning, you can tune a smaller undersized reactor (to a point) to meet demand of a larger tank, but media changes will be frequent, as will CO2 fills
More appropriately or even larger reactors can be tuned to smaller demands, I ran a Korallin 3002 (30" tall) rated for upto 800ga on a 58gal SPS tank, bubble count was rediculously slow though.
second stage chambers can definitely help buffer pH of effluent as well as adding in a few more degrees of DKh onto the effluent too! neccessary? no, but it's nice if you have the space.