I use MTG and put my mares tail in a braided tail bag, her tail has gotten much thicker and longer, doesn't happen overnight but you would likely see a big difference by spring. I leave the bag on her tail for about a month at a time in the winter since she's not swinging it around as much as she would in the summer. Every month I take it out and slowly brush through it, put more mtg on it and wrap it back up, and in the summer I re-do the bag every two weeks. Mtg is a bit smelly and messy, but I really think it works. You can just put a bag on it with no extra products and that would also work.