Technical side conversation split off of this topic. JasonLion
I have been told by manufacturers of sand filters that sand "movement" and "agitation" especially during backwashing, will slowly and eventually smooth the rough edges of the individual sand grains causing them to lose some ability to filter out particulate matter. Whether that is absolutely true, I don't know. They also suggest that 5-7 years is the general recommended time to replace the sand. For what it is worth, manufacturers of sand filters don't necesasarily sell sand in the after market. I have experienced many water problems that are due to poor filtration, and at the very least, contributes to some water problems. Circulation and water flow is also another issue to consider.