Coraline on glass but not on rock

[Aquattro]

With that much rock and sand, I'm surprised you have any NO3. What do you feed? Frozen foods can add a lot of nutrients. Is the sand bed old(over 3 years)? How about the rock? Is it clean on the surface? Try using a turkey baster to clean it off and open the pores, the internal (anaerobic) surfaces are where the NO3 is broken down, make sure the water can get in there.