can't maintain alkalinity

[fishface]

so i've got a marinetech ca reactor and it pumps out the Ca...levels are constantly high...as in > 450ppm. alk however is constantly low...like < 7dKH. i've tried every combination of adjustment that i can make with effluent and co2 but cannot maintain my alk. salt mix i use has low alk (6.5-ish) so i've been boosting this before it hits the tank but am i under any illusions that this reactor should be able to maintain alk levels?

appreciate any input...

ps my mg levels are good at around 1380-1450ppm.