8 US gallons is pretty small to be a community tank with 7 fish. If it is UK gallons that's not so bad. It's possible that the high bioload is contributing to his lack of re-growth. Alternatively, he might be stressed by a lack of personal space, or may even be suffering from being nibbled at by the other fish. What species are they? Personally, I would upgrade your tank size, or keep your betta separate.
It can take ages for proper fin regrowth. My advice is:
- Don't treat with chemicals or medicines.
- Do keep the water nice and clean.
- Do feed the betta a high-quality diet, such as New Life Spectrum Pellets, or even some live food.
- Do acquire some Indian Almond Leaves - they really help bettas to thrive.
- Turn the temperature up. Keep him between 80 and 82F. This will speed up his metabolism and encourage new growth.
- Do keep his stress levels low - give him lots of lovely hiding spots.
- Don't expect his fins to return to all their former majesty. Some fish never regain their original finnage.
Fin rot can, in some fish, be quite hard to defeat. Even when you think you've beaten it, it can suddenly come back aggressively. My first male VT battled it all of his life. Don't feel as if you are doing something wrong - sometimes, it's just not easy to heal. Follow the advice I've given, and you'll hopefully see some improvement, but don't beat yourself up if his fins never look like a show fish. :)