I have an IG 20,000 gallon pool, using a sand filter. Last year I didn't have to backwash very often once my pool was up and running. I did it maybe once every three weeks or so when I noticed a lot less water flow through the system or a much higher water pressure on the gauge. This year, I'm having to backwash about once a week. It's really not a problem to do so, but I'm wondering about the change.

Also, I remember reading once that when you backwash you should do so for three minutes and then rinse for 30 seconds. Is it necessary to backwash for three full minutes even if the water in the little viewer bubble looks clear after only one minute? I don't want to send my pool water down the drain for two extra minutes if I don't have to.

What do backwashing and rinsing do, exactly? How do I know when to add or change filter sand? I changed it two years ago, BTW. Does it need "topped-off" every once in a while?