hardened substrate? cause?

[bluetang]

This is commonly called "Lock Up"
Are you running a reactor? As said in the above post, it could be too much alk and calcium. I've been through this with a friend of mines 90g tank. The entire sand bead locked Up and had to be removed. We then smashed up the solid blocks of argonite sand with a hammer, bring it back to a granular consistency. Terible job... Keep up checking your levels.
Rob