When selecting the grain size of the sand to be used in a reef tank, there is no right or wrong choice, but simply choices that are more or less ideal for a particular situation. Finer grain sand provides a greater amount of surface area for biological filtration, but at the same time it can be blown around by the water current rather easily. Larger grain sand, on the other hand, is great at resisting water flow and typically stays in place. It’s big downside, however, is that detritus can accumulate in between the grains rather easily, and various algae can grow on the grains themselves. If you have a hard time making up your mind as to which grain is right for your setup, you can mix various sand sizes to best suit your needs. Use finer sand as the primary substrate and fill in the open areas with larger grain sand. There are plenty of choices of grain size on the market, so find one that looks best and fits your flow patterns the best.