DSB, sand do provide a lot more surface area compared to rock. Rather sand then. Dry skimming, you set the water level inside the skimmer lower, depending on skimmer type, either lower the outlet, or open the restriction. So the skimate is less watery and darker, Very thick soup like feeling. Wet skimming is the water level inside skimmer higher, easier for bubbles to reach the top. So the skimate is a lot more watery, normally more green and like a green tea.