It may be possible to get a filter cleaner solution and add it while backwashing, as the previous poster suggested.
I also would suggest replacing the sand but bleaching will have nearly the same effect. Sand used in filters should be replaced every year, however if they haven't used it in a while it may still be good sand and need a good backwash.
Worst case scenario is you have to refill your pool with more clean water and add new sand, I would try it before just to see what the results are, learning experiences are fun if they are at your leisure :P
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