If you are running an aquarium with lots of water flow, chances are you have spots in the sand that expose the bottom of the aquarium. The massive amounts of water movement churn up the sand, and over time the “bald spots” will just develop and stay there. No matter how many times you adjust the direction of the flow or pack sand into those spots, the current will just remove it. To solve this issue, try using a heavier grain substrate. The heavier grain sand will stay put, for the most part, and will help hide those spots where the glass shows through. Even if you don’t like the look of the larger grain sand, it shouldn’t affect the overall look of the tank, provided it is used in small areas.