Hello and thank you for your inquiry.
When I have seen something similar to this occur it was due to an imbalance in the calcium and alkalinity levels. I would suggest starting by testing what their levels are at and if they are in the proper balance I would recommend dosing to help balance them. Ideally calcium should be around 380-450 ppm and your alkalinty should be around 8-12 dKH. If your calcium levels are on the lower side of the optimal conditions your alkalinity should also be.
You can also try stirring up the sandbed some during water changes, just the upper level though as you don't want to distrupt any biological activity within the sand.
If your levels are where they should be, I would recommend giving us a call (1-800-566-3474) or email us at firstname.lastname@example.org so we can get more information about your tank.
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