Just for more background, what size tank? How long has the tank been going? What temperature is it? Do you have plants or other hiding places?

petioaris -- the tank is a 2.5 gallon and i've it up and running for a little bit over a year. the temperature is around 75ish (i have a hood light to keep the water warm) and yes i have one plant that he would always hide under and swim around when he was healthy, and a small deorative shark.

Is the filter medium new or has it been going for awhile? Temperature could be higher, say 80 F. In my opinion, Tetracycline is the best med for white cottony growths and bacterial infections. It's a little messy and ugly but worth it.

i've had this filter for about 9 months now. i'll try the tetracycline and up the temperature a bit and let you know how it goes. thanks =]

And max out the aeration as well! Wait at least one full day before doing any water changes. The meds need that much time to work.

How often do you change the water in this tank? When did you do it last?

A 2.5 gallon tank is too small for a filter to be effective, you should be doing frequent water changes instead. Have you tested the water to see how much ammonia is in it?

I respectably disagree with the assessment that a 2.5 gallon tank is too small for a filter to be effective. A small sponge filter would be adequate for the tank, once cycled. Frequent water changes stresses out fish and they become more susceptible to ich and bacterial infections. That's too much changeover of water.

Have you tried adding a small amount of aquarium salt to the water? This will help with gill function in fish. Also, try to change your filter more but leave algae on the overflow lip. This will help remove microscopic debris particles from the water and will keep the fish healthy and water more clear. As mentioned before aeration is important. Water must be circulating freely to oxygenate the water properly. If you are feeding a pellet food you may want to try a small flake such as Wardley's.

It sounds like the dredded Columinar Bacteria. (common in bettas kept in pet store cold water bowls in their own you know what). Really a bacteria, and it goes internal.

Hard to spot until amost too late. Slight scale discoloration, when gets fluffy, already inbedded in tissue, gills even interior. See links below.

But it is horrific on their immune system and meds are hard as well. Increasing salt levels I have used now exclusively, and then it is hard to truly cure. I resuce bettas at the end from pet stores. About 1/3 make it but again they are in horrible shape. It is in the water esp warm summer water and some strains kill overnight. (same bacteria in man is the flesh eating bacteria.).

I use small penn plax filter as above they do make them for smaller tanks. My bettas are in 2.5 to 5 and up depending on each preference. (some did horrible in larger so back into smaller where they are fine).

Plants are floating Anuibus or on wood. I do frequent but very small water changes, up to once a day, depending on looks. (breeders do full each day, but they are in very small spaces). I agree with not doing too much too often, but for me about 1 tenth change each day or every other with larger if in hospital tank (about one inch or so in 2.5) reduces this bacteria. I don't over feed, and feed vegetable as well. (very small pinch of food am, veg pinch to purge early evening).

Any left over food is turkey basted out after they are done scrounding around.

IT is really hard to treat. Goes into the gills, into the muscle tissue, arteries, so on. Systemic.

If you do wish to try, try salt build up. change water each day with gradual salt build up. Up to many teaspoons per gallon with betta (has to be gradual build up. One per gallon per day). I have also used hydrogen peroxide dips and tretment (takes great skill and proper doses) but have found the salt works best. It has mutated in fish breeding set ups so meds don't often work well anymore. But diff strokes for diff fish/folks.

Adding medicine to foods helps as well. Best Sherry

In case fish passed and for future readers here are some links on columnaris (by it's various names, a bacteria not a true fungus).

http://www.aquarticles.com/articles/ponds/Kebus_Salt_Treatments.html

PDF file gov source:

http://aquanic.org/publicat/usda_rac/efs/srac/479bfs.pdf

(great photo and many thoughts on treatment)

Flexibar verses true fungus, with diff tretments (I would like to be able to afford meth blue (or even find it)

http://www.americanaquariumproducts.com/Columnaris.html

http://www.fishdeals.com/fish_diseases/parasites/

Cotton Mouth - Identification & Treatment
(Also known as: Columnaris, Cotton-Wool, Cotton-Mouth, Flexibacter, Mouth Fungus)
Much has been written on the topic of stress & disease, below is summary to help guide you throughout Cotton Mouth prevention and identification. Please feel free to send any comments or suggestions to Ask An Expert.
Name: COTTON MOUTH
Symptoms: Loss of appetite
White spots on mouth, scales, and fins
Cottony growth on mouth
Fins disintegrate beginning at the edges
Looks like a small cotton ball

Symptoms of Stress & Disease
Cotton Mouth General Description

Mouth Fungus is so called because it looks like a fungus attack of the mouth. It is actually caused from the bacterium Chondrococcus columnaris. It shows up first as a gray or white line around the lips and later as short tufts sprouting from the mouth like fungus. The toxins produced and the inability to eat will be fatal unless treated at an early stage.

Cotton Mouth Treatments
Hard to treat, but not impossible. Like any disease best caught early. There are commercial products available to treat cotton mouth. Also change the water daily 20% and use a regular dose of salt in your tank.

Cotton Mouth Prevention
Usually this comes in with the fish directly from the pet store, or shipment. Because the bacteria thrive on organic wastes, it can be controlled by regular water changes and vacuuming of the gravel. Proper diet and maintaining good water quality in general will keep the fish from being stressed and therefore susceptible to infection.
http://freshaquarium.about.com/cs/disease/p/columnaris.htm