Are there NO instructions on the tetracycline bottle? Usually a full treatment cycle is completed before a waterchange as long as the water tests good. The treatment cycles are 5-7 days depending on the manufacturer. Sick fishes especially need clean water. I would remove the carbon if you put it in and use wool or some other innert mechanical media. Carbon will remove the medication with most brands but Maracyn/Virbac. Usually at the end of a treatment cycle, 30% water change is done, and the carbon is run a day or two before repeating the treatment. Again it differs with different brands.
The problem here is the uncycled tank
Do you have any decor that you can spare from a well and established tank? Oh well! the tetracycline would probably kill the good bacteria any way. Just do a good clean, I think, on the usual day or if the tank appears scummy. Try not to leave uneaten food in the tank.
Call the store where you bought the medication and ask for administration instructions.
carbon/no carbon
frequency and amount of dosages
length of treatment cycle
Did you say you have a medicinal dose of salt added?
I keep asking about salt dips and Betta, but no one is answering me. They are very effective on some fish with culimnarus and fungal infections.
You are getting a crash course in quarantine and the standbys needed.
Here is an excellent site to put with your favorites:
Well I can't leave that link. There is a Forum around the corner at the other end. So here is a copy of the article. After all, plagerism is better that friendly sharing.
Columnaris Information and Symptoms
By Rachel Hunt (Mushi)
Image © DanelleColumnaris is commonly mistaken for a fungal problem, it is NOT a fungus, it is caused by Flexibacter bacteria. Columnaris can be particularly difficult to diagnose as there are many symptoms associated with it.
Symptoms can include:
White, gray or clear stringy-looking "fungus" hanging off the betta's body or fins. Again, this is not actually fungus, but the Flexibacter bacteria.
White or gray patches that look like mold or a slime covering, usually on the body (and most commonly around the dorsal area).
White "pimples," generally around the chin and mouth area.
White or gray tufts of "fungus" (often looks like cotton candy) anywhere on the body, but especially around the mouth, gills, or edges of scales.
Lesions anywhere on the body, generally beginning in the dorsal area. Redness and swelling are not uncommon.
In the later stages of infection, the bacteria will begin to eat away at the fish's scales, often leaving behind a red or brown looking bloody area surrounded by fungusy-looking tufts of bacteria.
The Betta may also have clear, stringy feces and may be lacking appetite.
Columnaris is highly contagious. Any fish in a community setting showing signs of Columnaris should be removed to a hospital tank and treated seperately. A watchful eye should be kept on the fish remaining in the community for signs that they may be developing the disease.
Columnaris Treatment
Columnaris can be extremely difficult to treat, so it is recommended that medication be started as soon as the disease is properly diagnosed. As suggested above, remove the fish to a hospital/quarantine tank if it is normally kept with other fish. High temperatures will accelerate the course of the disease, so it is best to keep the affected fish at around 75-76 degrees F to slow the progression of the bacteria.
Columnaris is a gram-negative rod bacterium, and should be treated with an antibiotic that is effective against gram-negative bacteria or a broad-spectrum antibiotic. Kanacyn (Kanamycin sulfate), Spectrogram (Kanamycin sulfate and Nitrofurazone), Tetracycline, or Furan 2 (Nitrofurazone) are all good choices. I have also heard of people getting good results with medicated foods containing oxytetracycline, but if your fish is having problems with its mouth due to the infection, this may not be an option.
Columnaris Prevention
Columnaris is often a sign that your water conditions are not optimal for the fish. To keep water quality optimal and reduce the risk of your fish contracting Columnaris:
Avoid rapid, frequent temperature changes.
Do not overstock your tank. If you do overstock, be aware of what you're doing and keep up with the tank maintenance.
Do not overfeed. Rotting food adds to tank pollution, if the fish don't eat it, clean it up before it causes problems.
Feed the fish a variety of foods so that it does not have any nutritional deficiencies.
Always quarantine new fish for at least 2 weeks to reduce the chances of intruducing a disease to a healthy tank.
Water quality is the most important step in preventing Columnaris, tank maintenance is key.
