In beta fish fin rot is a common disease. Fin rot is caused by different types of bacteria, often occuring as a result of poor water quality.
Fin rot is also contagious.
The symptoms include the fins turning white, or shrinking. Often the fins become frayed, and the base of the fins become swollen or enflamed. Sometimes the entire fin almost disappears.
Often cotton like substance will appear on the fins when fin rot is present.
There are many ways to treat fin rot. Since the primary cause of fin rot is poor water quality the first thing you should do is completely change the beta fish water, and clean the tankthoroughly.
to read more about changing beta fish water.
Next you will want to add a little Mardel Maracyn 2. Maracyn-2 is a broad-spectrum antibiotic (minocycline) tablet often used to treat fin rot.
Finally some aquarium salt can be added to the tank. If the treatment is working you should notice a significant improvement in your beta fish in about 2 days. Within a week there should be no signs of fin rot left.
If treated correctly fin rot will disappear and your beta fish should return to a normal healthy state. However if left untreated your beta fish may become very sick and eventually may pass away.