Fin Rot is arguably the number one issue you will have to deal with if you keep any number of bettas for any length of time. Fin Rot is a bacterial infection of the fins, usually brought about by poor water conditions and/or stress that causes the fins to deteriorate. Fin Rot can also set in when a fin is injured and the water is not kept clean enough to avoid infection of the injured fin.
Symptoms can include:
Small holes (pinholes) anywhere on the fins.
Fins that seem to be growing thinner or more transparent.
Fins that seem to be "fraying" around the edges.
Fins that seem to be breaking off in large chunks.
Slimey looking areas, usually on the tips of fins, that seem to "eat away" at the fins over time.
Fin Rot in and of itself is not contagious, but if you keep fish in a community tank you may end up with several fish contracting the infection. This is because the bacteria that cause Fin Rot are opportunistic bacteria - poor water conditions stress fish out, and when fish become stressed, their immune systems become weak. Weak fish are not able to fight off illnesses that healthy fish easily can. So if you have poor water conditions or something else causing stress in the occupants of your tank, you may have an "outbreak" of Fin Rot.
Quick Medication Dosing Tip Since most medication dosing directions are meant for larger tanks and some medications come in tablet or capsule form, it can be difficult to dose a small container. There is a trick to doing this.
If your medication calls for one capsule to 10 gallons, grab a small container that can hold water. Using a measuring spoon, measure out 10 teaspoons or tablespoons full of water into the container (it doesn't matter which you use as long as you use the same unit of measurement the whole way through). Dump the capsule or crush the tablet into the measured water. Then dispense the medicated water into your isolation container - if it is a 1 gallon container, one teaspoon/tablespoon will be the correct medication dosage. This method works for any size treatment container, just be aware that most medications lose effectiveness after 24 hours in water so you will have to discard any unused medication using this method.
Fin Rot Treatment
Even though Fin Rot is not contagious, it is still a good idea to isolate a betta that you are attempting to treat for Fin Rot. There are several reasons for this, but the main reason is that it's much easier to keep the water absolutely pristine when you have a fish in an isolation container. The secondary big reason (maybe this should be first) is that many medications used to treat fin rot can kill your biological filtration, live plants, and may be harmful to other animals in your tank (depending on the medication and the other animals). It's also just generally a good idea to isolate any sick fish when treating them so that they are not getting picked on and there is no chance of passing any secondary infections on to other fish, as well as making it easier on you to monitor the health and progress of your fish.
Enough of the "isolate your sick fish" lecture (you're probably sick of it by now if you've been through other parts of this site, right?). Fin Rot is caused by gram-negative rod bacteria. Fin Rot can be easy to treat if you catch it early enough and maintain your water quality pretty well, or it can be a complete pain in the bottom if you don't catch it early enough or allow your water conditions to fluctuate over long periods of time (you know, we all get lazy every once in awhile).
If you catch your Fin Rot in the "pinhole" or "just starting to fray" stages, you may have good luck getting rid of it by simply increasing the number of water changes you do. If your betta is in a small, unfiltered tank, simply doing daily 100% water changes for a week may reverse the beginning stages of the illness. Adding 1tsp of aquarium/rock salt per gallon of water to the isolation tank along with doing daily water changes may also help take care of these bacteria in the very early stages.
Important Medication Note Many Petstores will try to sell you a product called Melafix or Bettafix for the treatment of Fin Rot. In my personal experience, neither of these products (which actually have the same active ingredient, Melaleuca or Tea Tree Oil) will help with the treatment of true Fin Rot. They may help regrow fins damaged by Fin Rot once the bacteria is killed off, but they are not the right type of medication to actually "cure" Fin Rot. Treating with either of these products may stop the advance of the rot, but once you stop treatment it is almost guaranteed to come back. Don't be fooled into purchasing this medication as treatment for Fin Rot, although you can purchase it to use after you've cured the rot and just want something to prevent growing fins from being re-infected. End of lecture!
If you notice that the Fin Rot is getting worse or is way past this stage when you notice it, it's time to bring out the big guns. Because Fin Rot is a gram-negative rod bacterial infection, you want to treat it with either a good, broad spectrum antibiotic or an antibiotic that specifically treats gram-negative rod bacteria. My favorite medications to treat fin rot are Jungle Fungus Eliminator and Tetracycline. If you are unable to find either of these medications, other options are Maracyn II (you can use it in combination with Maracyn if you want to cover all the bases) and Kanacyn. Simply follow the dosing directions on the medication, keeping in mind that since you are treating in a small container you will have to do full water changes and add new medication instead of doing the partial changes many medications specify.
I personally like to continue treating with Fin Rot medications until I see good, solid fin regrowth (about 1/8"), even If that means I treat longer than the treatment schedule calls for. I do this because many times medications will stop the progression of the bacteria, but may not kill them all right away. If you stop treatment before all of the bacteria are killed off, odds are the Fin Rot will return as soon as you stop medication. Fin regrowth is an excellent sign that the bacteria have been taken care of.
If you run through 3 treatment cycles (usually about 15 days for most medications), you have kept up with the water changes, and do not see any improvement in the condition of the fins, you may want to switch to another medication. Before switching medications, give the betta a "med vacation" of about a week, changing his water (without adding any medicines) every day during that week before starting a new medicine.
Sometimes you will get a stubborn case of fin rot that nothing seems to help, or a betta that just seems prone to contracting Fin Rot over and over no matter what you do. If you run through several medications and nothing seems to help, you may want to just try 1tsp/gallon of rock/aquarium salt and daily water changes for a few weeks and see how he does. Sometimes we can so overmedicate our fish that the medications themselves start stressing them to the point they get sicker and sicker and stopping medications might be the most humane thing we can do. I've had extremely Fin Rot prone bettas do excellently with just daily water changes after all else had failed.
Fin Rot Prevention
Because Fin Rot is an opportunistic disease, keeping your betta in optimal conditions will greatly reduce the chances of him getting Fin Rot. Always keep a betta's water so clean you could drink it (not that you'd want to, but...). Always keep bettas at a good, steady temperature. Fluctuating temperatures or temperatures that are too cold (under 72 degrees F for long periods of time) or too warm (above 82 degrees F for long periods of time) can greatly increase the chances of a betta getting ill. Feed a well-balanced diet to avoid nutritional deficiencies that can weaken the betta's immune system.