Vinegar and baking soda works pretty good, but is a bit drying, so you need to shampoo and condition afterwards.
Wet the tail, add baking soda (just regular baking soda), rub into the tail, then pour white vinegar over it. [Might want to put the tail in a bucket if you can to contain the liquid.] The vinegar and baking soda will react causing bubbles and effervescence. Rinse the tail thoroughly, then wash with shampoo and conditioner. It might take a couple tries, but I wouldn't do too much in one day, as its drying.