I'm going to start with identifiying what's going on diatoms/dinos.
I'm also going to siphon off the top layer of my sandbed and 'gravel clean' the rest.
Then I will add back a top layer of new sand that has been THOROUGHLY rinsed to remove any particlulates.
This will ensure I can keep my sand clean in the future.
I think what's going on is I didn't wash my new sand well enough so the fine particulates are trapping detritus.
Also, I think the diatoms are causing my sand to calcify.
If it is Dinos, and cleaning my sand doesn't solve it, I'm going to consider the Metro method being discussed on R2R.
Dinoflagellates - dinos a possible cure!? Follow along and see!
It's over 230 pages so be prepared for a night of reading.