For everyone in here having fin problems and you've tried everything...
Firstly, make sure your betta is on a food with high protein. Frozen bloodworms that have been thawed are an excellent source, and a good pellet such as Atison's Betta Food or New Life Spectrum. Also, you've gotta have a heated tank if you plan on treating these advanced fin-rot/body rot problems. Buy a big rubbermaid tub that's about 4-5 gallons from Wal-Mart and stick a heater in it. Instant QT tank.
1. First treatment to try is 100% daily water changes with AQ salt, 1tsp/gallon, and water conditioner. Do this for 10 days in a separate tank than their usual. If no improvement...
2. Indian Almond Leaf or Oak Leaves. Natural tannins. No improvement?
3. At this point, you don't want to try everything under the sun because it could weaken them further. As Adastra stated, you could have naturally bad water quality. It's important to take your water to your LFS/LPS and get it tested. I would start using maracyn I and maracyn II in conjunction with each other.
The powder packets are made for 10 gallons. What you want to do is get a syringe (like a medicine syringe used for babies) and dilute this powder in a bowl or cup. 10mL of water, mix it with the packet. Take 1mL of this new solution per gallon of water. You'll want to do this for both maracyn I and II. Daily water changes, follow treatment according to packet.
If this doesn't work, then... Well, Adastra mentioned fin clipping, and I found an excellent article on it once.. But there are risks. Most people won't do it unless their betta is literally dying.