Sand bed hardening and Ca running high

[rsp109]

Just at the title says, the surface of my sand bed keeps hardening (not concrete hard, but still clumpy). I keep manually breaking it up to prevent development of large anoxic regions, the sand does turn slightly darker under these areas though there's no sulfur bubbles since I keep disturbing it before it goes unchecked. My ca is at ~480 and alk at 10 dkh with SG of 1.026, even though my salt mixes to 440 and 12 at 1.026. I don't dose anything and I've been stocking the tank with sps frags hoping they'd bring down the Ca but no dice (though they're growing nicely!). I have a dsb varying between 3-4 inches, my guess is that it's doing it's job too well. No matter how much I feed ammonia, nitrite, and nitrate all stay at 0; ph of 7.9-8.0 around the clock. Any ideas on what's going on? I need to get a magnesium test kit and check my stats to see if I could help speed up coral Ca metabolism, but they're growing quite fast already so I don't think that's the problem.