I had a problem with sandbed clumping a many years ago when I was using kalkwasser. What reading I did indicated it was either precipitation or high levels of bacteria aggregating the sand together. I stopped using the kalk and broke up the sandbed, I don't remember where it went from there though as I likely had other issues at the time. There is a "test" for this by which you put a clump of sand in bleach. If it falls apart its bacteria, if not its precipitation.
I find it odd that every type of media clumps though, something odd is definitely going on in there.
I'm just working my way through this, maybe have a look and see if anything fits?
http://www.reefcentral.com/forums/sh....php?t=1864522