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I find that when I have an algae infestation and vacuum the bottom after it settles, the pool is totally stirred up and green later in the day. This means that it takes a lot longer to filter and backwash all the algae out taking over a week. I seem to detect a pale green color entering the pool from the return fitting even as I'm vacuuming? Is my filter old? I was told that I shouldn't need to change the sand and haven't done so in a couple years. Do I need to use a clarifier?
You need to be killing the algae with chlorine prior to filtering it out. Bring your chlorine levels up to "shock" level per your CYA reading and the best guess page. Consistently keep your chlorine level there until you no longer "lose" chlorine between sundown and sunup the next morning.
My understanding is also that you never need to change the sand in a sand filter. If it's blowing stuff back into the pool maybe there has been some other internal malfunction? Someone else with sand filter experience will have to help you there.
Take a couple of handfuls of DE and put them into your skimmer. Watch your return jet. If you see it shooting out of the return, then you may have a broken filter. If not, then, your pressure will probably rise substantially within a few minutes. If it rises 8-10psi over your clean filter pressure, backwash the filter.