buffer problem

Hi folks. I haven't visited the NG for a while, but I seem to be having a
little bit of a problem, and thought I'd give it a try. I'm having trouble
maintaining dKH, but I'm not sure if it's a big problem. I'm recently
having to add buffer more often to maintain the desired level. It drops
from 10-11 to about 8.0-8.5 within four or five days. My reef is well
established, up and running for approximately three years, with healthy
corals and good SPS growth. It's a 90 gallon with a refugium w/skimmer
set-up. Water parameters are all good and stable, but the dKH seems to drop
too quickly. After more than a year and a half of working on establishing
what I thought was a good routine at dosing and keeping dKH and calcium
levels balanced and stable, I got a little lax and didn't test very often.
I really didn't think I had a problem with the buffer until I experienced a
small "snowstorm" recently that was a result of my trying to bump the
calcium up too much while the dKH was too low. I was able to get everything
back to acceptable levels pretty quick, but I started watching the dKH
closely and noticed that it drops too quickly. There's probably a fairly
heavy demand for calcium, but I've always been able to maintain it between
410-420 with kalkwasser for make up water, and by dosing a small amount of
Kent's Turbo calcium about once a week. I use Kent's Superbuffer for the
alkalinity. There's a mix of corals, with some SPS, and a couple of clams.
I do have a lot of rock relative to the size of the tank, and it's
completely covered with coralline, as is the back glass and the overflow. I
also maintain magnesium at approximately 1300-1320 by adding it at one week
intervals. I use Instant Ocean and do a 20% water change every-other week.
I used to drip the buffer at the same time I dripped the turbo calcium (In
separate containers mixed with RO water), but now I have to add buffer more
often. I don't test for nitrates anymore, but I've read that high nitrates
could cause problems with dKH. There does seem to be a small amount of
detritus, but the bio load is pretty light, and I don't overfeed my fish.
All of the water parameters are fine and stay pretty stable, except for the
dKH. Again, the corals are healthy with good growth, and the clams are
healthy and show good growth too. For a long time I was able to keep levels
balanced using the Superbuffer and Turbo Calcium, but I'm now considering
switching a two part product to see if that will help stabilize things. Do
the two products I'm using create some kind of imbalance? Would I be better
off with a two-part product? Please let me know if I need to give more
information about my water parameters or about my system. Any help is
appreciated.
Royce