DSB question
 

Yesterday I did a water change in my system. 77 gal display and 80 gal sump. In my sump, I have a refugium with a DSB. Over the last couple months I've noticed strands of chaeto laying on the sand and thought I'd clean it up a bit. So I took my siphon tube and ran it lightly across the sand in the hopes of just vacuuming it all up. I noticed though in one corner of the refugium that the gravel had turned into almost solid rock. I understand the dangers of screwing around too much with a DSB but with utmost caution I jabbed it a bit to break it up back to sand again. Does anyone know why this happened? I'd assume when it gets like that water can no longer sift and flow through it then I'm really in trouble.

What causes sand to stiffen up like this and should I be getting little critters to mix the surface a bit? I've read that is a bad idea but I can't think of how to prevent this from occurring again.

Any help would be most appreciated

I had a 6" sugarfine DSB in a lighted gium with cheato and it caked solid in @ 6 months, so I removed it and did an 8" sugarfine remote DSB in an unlit bucket and it did not cake in 6 months so I'm thinking it is something to do with the light or the algae, however, I eventually decided that they weren't worth the trouble and weren't controlling my nitrates and removed it.

I've used mildly deep sand beds over the years, no issue. But my last tank, set up pretty much the same as the ones previous to it, had the sand solidify in about 4 months. I mean solid. I took the 24x48 inch sandbed out in about 7 pieces...I never figured it out. I have 2 inches of the same sand in my current tank, no issues in 18 months...

dsb
 

possibly calcium is gluing it together

A spike in alkalinity will cause sand to bind like that. A spike you may or may not have known about.

Take a small clump of the sand out and put into a container of bleach and water. If the clump breaks up into the original sand particles then it is bacteria or algae holding it together. If it stays in the original clump then you have calcium precipitating out for some reason

Hmmm, cool, I'll try that.
Thanks for all the responses so far.