If the denture cleaner doesn't work and the decanter isn't etched, which would be impossible to remove, try an enzyme based laundry detergent or stain remover. This might work since the stain is organic. The denture cleaner should also work against organic stains but there are different types that work in different ways. I have also read that you can use sand as an abraisive cleaner in decanters. Just add sand and water and swirl around. I wonder if you put the detergent or enzyme booster, water and and sand in the decanter and set it on something that vibrates like a dryer or refrigerator if that would remove the stain.