I don't know of a single salt that has dkh 12+. My guess is your test kit is way off or you are doing something not quite right executing the test or even reviewing results.
If you don't have SPS, it's probably not a huge concern anyway. 12 is def high, but not likely going to cause a huge problem in he short term. Assuming it is real, I would do as others suggested and just let it naturally come down.
Pending your selection of corals, u def need to look into dosing needs. In most cases, you cannot rely on water changes alone. Even tanks with few corals, but alot of coralline algae will consume a fair amount of alk and calc
Sent from my Nexus 6 using Tapatalk