I got this off of healthybetta.com
CAUSES:
Fin and Tail Rot is a common problem betta owners face. It is most often caused by dirty water. If the water is left unchanged, their own feces and uneaten food particles begin to break down, causing bettas to become stressed and more susceptible to the bacteria naturally present in their tanks that causes fin rot. The best measure against this is to always keep your tanks clean. A betta that has recently been under a high period of stress, may also develop this illness. Sometimes regardless of how well a betta's tank is maintained, the illness still strikes for unknown reasons.
DIAGNOSIS:
Early detection of the illness is the best to try and prevent further fin damage. First know that clear or whitish tips on your betta's fins and tail is fin re-growth, which is a very good thing. If your betta has dark brown/black tips, holes, or tears in his fins that appear to be getting worse, they most likely have fin and tail rot.
In a worst case scenario, the fins and/or tail could be falling apart, shredding, or be completely missing.
TREATMENT:
The first thing to do is a complete water change. Make sure you wash everything with hot water (no soap).
Medication(s):
Fin and Tail Rot is a bacterial illness, so you will want to make sure your medication is intended for this purpose.
One medication that is often used is Jungle Fungus Eliminator. For 1 gallon of water, use 1/4 teaspoon. For 2 gallons, 1/2 teaspoon and so forth. Treat the tank for 4 days then do a complete water change. If fin rot is still present, repeat dosage.
Other common medications that have been successfully used are Tetracycline, Maracyn and Maracyn-Two combined together, along with sulfa-containing medications.
Click HERE to view our Medications Made Easy page that provides medication dosage on a 1-gallon tank basis.
The use of aquarium salt:
Optional: Adding aquarium salt (or a bit extra if you already use it as a preventative) to a betta's tank when they have fin rot may also speed up the healing process. The usual dosage for preventative measures is 1/4 to 1/2 teaspoon per gallon of water; and up to 1 teaspoon per gallon for ill fish.
ONCE HEALING HAS BEGUN:
It is important to know that once you begin to see whitish/light colored re-growth on your betta's fins, it is very delicate. The slightest negative change in water conditions could cause the fin rot to return or the fins to shred. Continuing with prompt water changes will help in the healing process. Some may recommend treating with Melafix or Bettafix once the fin rot has been halted to promote faster fin re-growth. The dosage for Melafix is 10 drops per gallon of water and Bettafix is 1/2 teaspoon per gallon.
Additional procedures:
Fin and Tail Rot is known to be contagious between fish. Always use separate nets for different tanks and wash your hands before and after handling fish or tank materials.
Clean water is always important for a betta, but is especially beneficial for any betta facing an illness. You may find it helpful to change your betta's water more frequently while you are treating with any medication.
Always keeping your betta warm, between 76-82 degrees, will also help them to feel more comfortable and raise their immune system.
In essence, fin/tail rot is bacterial, maracyn 1 + maracyn 2 works wonders, I've also had great success with the jungle fungus eliminator, almost anything that is a broad spectrem antibiotic. The real key to curing fin rot if crystal clean water and clean gravel, since its caused when a chewed or ripped tail/fin is trailed accross dirty gravel and the bacteria gets inside the wound. Good luck, he's gorgeous, let me know how it goes
