Hi, thought this might help you, it's from Hayward's manual for their cartridge filter:
"If calcium or mineral deposits are found, clean cartridge by soaking in a plastic container with muriatic acid added to water in a 1 to 1 ratio. Always use caution when handling muriatic acid. After cleaning, flush with water."
That would be a much higher concentration than your friend advised, so I think you'll be fine. You may want to add more acid . . . Wait for a few more replies though, someone here may have a better cleaning method . . . From my research on this board before the big crash, I seem to remember this being a pretty common method of cleaning cartridge elements. Be VERY careful with the acid, remember add acid to water, not the other way around!!
Hope that helps!
-SJ
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