Acrylic stain removal?

I have an old in ground spa that Im sure is acrylic vs fiberglass. I has been empty a while and leafs and clutter sitting in it have set bad dark black stains.
Can these be removed from acrylic?
I spoke with local pool supply and they suggested trying 4:1 dilution of muriatic acid on the stains and then polish. But at that time I thought it was a fiberglass spa. Anyway, I tried it and it had absolutely no effect on the stains.
Im struggling with what to do if we can make the shell presentable again. Its too expensive to replace.
thanks for any tips
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