I treated my stains with oxalic acid and it was an ordeal due to the amount of staining. Anyway, the best way to remove the worst staining from the deep end was to just throw clumps of oxalic adding in. Wrong! I have no stain but after two years I still have that hard oxalic acid clump spots. I have tried tubing muriatic acid to the clumps, bleach and excessive brushing and still its like chipping away at ice sculpture. how do i get rid of this clumps?