a 15 DKH/KH, Is not going to harm your fish as long as its Short term,
what you need to check is your Cal levels. what are they at, your CAL and (ALK / DKH / KH All the same thing) work together.. Usally if one takes over and goes too high it can cause the other to crash..
IF your Cal is normal and fine, then the best way to bring down your KH with 5% water changes, every other day, untill you get to your desired levels. When doing the water changes, test the water to ensure that its around 8KH.. if that is your desired level