low dhk problem

for the past 2 weeks ive been monitoring the carbonate hardness in my reef tank. everytime i test it, it comes out around 8-9. i would like it to be around 10-12 range. i use reef builder by seachem dosing the right amount for 3 days straight but it doesnt seem to have any affect on the dkh lvl. is there something that needs to be dosed with the reef builder to get it to maintain a higher level. any ideas on what i could do differently?

 

Do you have current magnesium and Ca values?

 

dont have a mag test kit but calcium is always around 400. so because my calcium is within range im assuming magnesium must be somewhere around what it needs to be. i do a 15% water change every other wed with red sea pro salt and no longer dose a two part system. just the reef builder power