Welcome back Melissa Leiter for another awesome post.
Bacterial infections are very common among goldfish (Carassius auratus) and koi (Cyprinus carpi). The two most common types of bacteria that cause problems in ponds are aeromonas and pseudomonas. Both are naturally occurring in ponds and lakes. They are not found in tap water since chlorine is added at treatment plants. Likewise when a new pond is filled using tap water this bacteria is not present. Once fish are added the bacteria arrive with them. The bacteria floating in the water does not usually affect healthy fish since they have a slime coat that protects them. However, if the water quality becomes toxic, the fish lose that protective slime coat and are very prone to infections. Aeromonas are more common then pseudomonas but both are responsible for numerous fish deaths. They both present themselves in the form of ulcers, external and internal hemorrhages, red streaking in fins, fin and tail rot and mouth rot. If left untreated most if not all the fish in the pond will eventually succumb to this infection.
Bacterial infections are treatable if caught early. When sores are first observed the water quality should be one of the first things to be tested since many bacterial infections are caused by poor water quality. Ammonia, nitrite, nitrate, and pH should be tested. If you do not have access to test kits many pet stores, such as That Fish Place, will be happy to test your water for free. If the ammonia or nitrite are high they need to be corrected before any medications can safely be added. Ammonia and nitrite are both very toxic to fish. Ammonia irritates gill tissue and eventually invades other internal tissue and organs causing them to shut down. High levels of nitrite readily oxidize with hemoglobin to form methemoglobin. Methemoglobin hinders oxygen transport through the blood causing the fish to sufficate. Microbe-lift is a good product to add live bacteria that will break down the ammonia and nitrite to make it less toxic to your fish. Pond salt will also help if nitrite is high since the chloride ion competes with the nitrite ion on the gills. As long as the chloride ion is at least 3 times but not more than 6 times the concentration of nitrite, the concentration of nitrite becomes reduced and the fish is able to retain oxygen.
If all the levels are in the acceptable range then medications may be used. There are hundreds of medications out there so choosing the right one may be difficult. Some medications that we have had success using in the store would be fungus eliminator, melafix, neoplex, kanaplex, sulfathiazole, and biospheres antibacterial. Fungus eliminator and melafix are more economical then the others if you are treating a large pond. Neoplex, kanaplex, sulfathidazole, and biospheres antibacterial are more acceptable to use in a hospital tank. Iodine dabbed directly on the open sore then coated with petroleum jelly to seal the wound can also be done but this should not be attempted by the beginner. Handling the fish can be very stressful and cause injury if care is not taken.
Ultimately prevention is the key. With that being said here are some simple things that can be done to protect your fish from contracting a severe bacterial infection; monitor water quality weekly, preform mothly 20 percent water changes, quarantine any new fish for a minimum of 4 weeks before introducing into the pond, feed a high quality staple food as well as fresh veggies like romain lettuce, zuchinni, and cucumbers, and avoid over crowding.