Betta Fin Rot, also called Tail Rot, is a much too common ailment bettas suffer from when you consider that it is 100% preventable. Like almost all other betta diseases, it is usually brought about by poor aquarium water conditions. The inferior tank conditions then lead to an uptick in the bacterial level in the tank, and these bacteria attack the betta fins and tail, giving them a ragged appearance.
Excessive, untreated betta fin rot can lead to extreme fin and tail destruction, affecting the way your fish moves about the tank. Left unchecked, it can lead to the contraction of Cotton Wool disease, one of the much more serious betta diseases.
The good news is betta fin rot, when treated properly, is easily combated, and your vibrant betta returns rapidly to its previous splendor.
Remove all fish from the tank, whether you see fin rot on them or not. Separate the apparently diseased bettas form other non-infected fish. This is a very contagious disease, and all fish must be treated at the same time. Wash all gravel, rocks and tank toys in warm non-soapy water. Clean out the tank itself vigorously, and add tertracycline per manufacturer's instructions to both the infected and apparently uninfected fish.
Once you have a clean tank with all gravel, plants, etc. ready to go, add the uninfected fish after monitoring to ensure they truly are clean. When your infected betta begins to heal, add him also.
The important thing to take from your run-in with one of the most common betta diseases is proper tank maintenance.
There is virtually no chance of betta fin rot in a healthy tank. That is how easy it is to kill this harmful bacteria. Clean your tank every two weeks or so, and it can be prevented.
Betta diseases are generally caused by poor tank conditions, which create a perfect breeding ground for bacteria. The fish then become stressed due to their poor living conditions, just as humans do, and their immune system begins to weaken. The strengthened bacteria then attack the weakened fish, causing any number of symptoms.
The key here is, keep the water level the correct temperature, with the correct salt balance (yes, salt is needed in a freshwater tank), and your fish will thrive, and live long, healthy lives.
Hope this helps,