There's something wrong with the filter. A properly sized sand filter (big enough) with a properly sized pump (small enough) *WILL* remove iron; I have done so many times! Unfortunately, AG pool filters are often over-sized, and the pumps are under-sized.
There are 3 things you can do:
1. Install a valve between the pump and the filter, and SLOW the flow through the filter.
2. Check the sand as mentioned earlier
3. Use polyquat; it helps stick the particles together.
2 - 4 quarts of polyquat per 10,000 gallons would be a good idea; it will keep indefinitely, and you should use some each time you add water. Use the Amazon links; that way you can be sure of getting undiluted (60%) polyquat, with no added 'ingredients' to cause problems. (Plus, it helps support the forum; we get a commission on referred purchases.)
Get *only* the 21 degree Baume / 31% acid -- that concentration is solely acid + water; other concentrations may have 'helpful' (NOT!) ingredients. Any place you can find that (Lowes, Ace, etc.) is fine.
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