I am amazed that nobody has mentioned backwashing the filter. Ideally this should be done on a daily basis. This involves reversing the flow through the filter bed and flushing the entrapped foreign matter to drain. Periodically the backwash can be beefed up by adding some hypo to the sump whilst backwashing. Then, as stated, the sand should last for 3-4 years. In the UK the sand used is classified as "sharp sand" or "river sand".
