Sand filters have a difficult time filtering out very small debris because often times the microscopic particles of dead algae and waste are smaller than the grains of sand themselves. A sand filter can also “channel,” causing pockets in the filter where water passes through easily, without going through sand. To prevent this, you need to chemically clean your sand at the end of each season with a designated filter cleaner, backwash frequently throughout the season, and use clarifiers to help the water remain crystal clear.