Muriatic acid bad for fiberglass hot tub?

Hi, I have an Artesian Oahu Elite Hot Tub. I was working to get the pH down and was using muriatic acid which is what I use for my vinyl pool. My local pool store said I shouldn't use that for the hot tub. I forgot what he said could get damaged, I think the steel on the jets? Regardless, I started using the Sodium Bisulfate that came with the start up chemicals. Is it safe to use muriatic acid to lower pH? Or should I stick with Sodium Bisulfate?

Thanks!