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This is relevant if you have multiple fish in one tank. Isolating the sick fish will prevent him from transmitting disease to the healthy fish. To prevent possible ammonia and nitrate accumulation, do not put substrate or live plants in the treatment tank.  Use a sponge filter in the treatment tank. It is less likely than a chemical filter to filter out medications, and is unlikely to stress your fish (like a strong power filter).  Place an enclosed water heater in the tank to maintain a water temperature that is recommended for your specific type of fish.  An air stone will help keep the tank well oxygenated.  The supplies for your treatment tank are available at your local pet store. Continue to monitor the other fish for signs for fin rot. Separate and treat them if necessary. In the United States, over-the-counter antibiotics (e.g., erythromycin, trimethoprim, and sulfadimidine) are used to treat fin rot. The antibiotic is typically added directly to the tank’s water. Carefully follow the medication’s instructions to ensure effective treatment.  Outside of the United States, you may have to obtain the antibiotics through your veterinarian. It is common to use a combination of antibiotics to treat fin rot, since the disease could be caused by several types of bacteria. Methylene blue is a non-antibiotic treatment that can also be used to treat fin rot. Depending on the severity of fin rot, fin rot can also be treated by trimming the affected fins to remove the damaged tissue. This medical procedure requires that your fish be sedated, so you will need to schedule an appointment with your veterinarian to have this done.  Your veterinarian will use sterile scissors to remove the damaged tissue. He or she will then treat the clean edge with antiseptic. Your fish may need additional treatment if fin rot has become extensive.  The fin tissue is more likely to grow back with minor cases of fin rot. Your fish should remain in the treatment tank until his clinical signs have improved. A good rule of thumb is to keep him isolated for about 10 days. During this time, look for improvement in clinical signs: normal color returning to his fins, healing of ulcers and lesions on his body, normal swimming behavior. Be mindful that the fin rot may be too severe for treatment. Effective treatment of fin rot involves both medical treatment and improvement of underlying conditions. Improving water quality is extremely important. Change the water to remove excess ammonia and nitrate from the tank and add fresh minerals to the tank.  If need be, change the filter and allow time for bacteria to grow that will help reduce ammonia levels in the tank.  Remove any dead plants to reduce ammonia accumulation. Reduce the stress in your fish's life by keeping him in his own tank, away from the other aggressive fish. You could also use a water heater and thermometer to keep his tank at the right temperature. Remove sharp objects from your fish's tank (e.g. gravel, sharp-edged decorations) to prevent further physical damage.

Summary:
Move your sick fish to a separate tank. Administer antibiotic treatment to your fish. Trim the affected fins. Keep your fish isolated. Address the underlying conditions.