My calcium reactor is 6 inches wide and 30 inches tall (I am making a bigger one!) That is all for my
250.
The best media to get is coral sand as I find I can run it to a low 6.2 resctor effluent ph and not have it turn to mush.
I am able to get 60 dkh out of the reactor! Some say you can get close to 80.
If that doesn't work then you need a bigger reactor, assuming that your chemistry is OK.
FWIW how are your magnesium levels?