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It would be good if you can issolate the fish as you don't want to be using a strong antibiotic like tetracycline in the main tank, as it will wipe the bacteria colony out, the hole did it start of as a red sore like an ulcer.
Tetracycline
Manufacturer: Aquatronics
A broad-spectrum antibiotic used in the treatment of gram-positive and some gram-negative bacterial infections. For infections of fin and tail rot, frayed fins, popeye, inflamed gills, mouth and body open sores/ulcers, livebearer disease, dropsy, Columnaris, and secondary infections such as fungal. Specific for livebearers and goldfish. Aerate the aquarium well when using. Useful for the control of some common bacterial diseases, including Aeromonias and Pseudomonas Genera and the Mysobacterial group.
Does the hole sound like this, not the writer of this information.
Ulcers- Ulcers are open sores located on the fish's body. Ulcers are sores on the body caused by bacterial infection. You need to make very sure your water quality is immaculate. Check your pH and Ammonia as well as your Nitrite readings. Ulcers usually are caused by the Aeromonas bacteria or more rarely Pseudomonas bacteria. The ulceration may appear with red abscesses, or red edges around the ulcer. In some cases it can become rather large and expose muscles. Symptoms: The fish has one or more open sores on its body or at the base of its fins. The sore has a red abscess or red edges. The fish may also have red fin edges or fin rot. They may exhibit lack of appetite and sluggish behavior. Treatment: Usually antibiotics are needed and this is done by a series of injections. ( A vet would need to see the fish and give you the necessary prescription). However this is not always the best option for individuals and a product by Aquarium Pharmaceuticals has a great product called Mela Fix that has shown great results in the healing of ulcers. It is easy to use and does not create issues with the water quality. (back to top) Koivet.com has more info on MelaFix here.