Bacterial Infections:  (The following is meant to be a very simple explanation of  why bacterial infections occur and how to treat them. For a more detailed discussion see “Koi Health and Disease” by Dr. Erik L. Johnson, D.V.M.)  The two most common bacteria that cause problems on koi fish are Aeromonas and Pseudomonas. Both of these bacteria are naturally occurring and are found in all pond and lake water. These bacteria are non existent in tap water because chlorine is added to kill any bacteria. Never the less, you can start up a brand new pond with tap water and within 3 weeks after the chlorine has evaporated, you will find at least a low level of these bacteria in your water. Normally, in a clean healthy pond, the levels of bacteria are so low that they will not cause a problem on their own. The outer slime coat or epidermis normally protects the fish from these harmful bacteria. The most common reason koi experience a bacterial infection is when their slime coat or epidermal layer is broken and the bacteria is allowed to infect the koi. These sores can grow very quickly if they are not treated.  

The most common reasons infections start

• Parasites can eat through the outer slime coat or epidermal layer exposing the dermis layer to the bacteria. As flukes attach themselves to the surface of the koi with their haptens, which resemble small fish hooks, they can actually inject the harmful bacteria into the koi.

• Scratching against a sharp object such as a rock can break through the slime coat and expose the koi to the bacteria.

• Poor water quality can burn off the slime coat or stress the koi making it more susceptible to the bacteria.

• Stress by itself through overcrowding or simply moving fish from one pond to another can sometimes be enough to allow a bacterial infection to start.

Treatment for Bacterial Infection 

Treatment can be handled in several ways depending on the severity of the infection.

• On small sores you can dry off the wound with a paper towel and then treat the wound with hydrogen peroxide on a q-tip. Let it soak for about 30 seconds before placing the koi back into the water. Repeat this procedure 3 days in a row.

• On larger sores after treating with hydrogen peroxide, again dry off the wound and apply Debride Medicated Ointment to the sore. Repeat this procedure 3 days in a row.

• Potassium Permanganate mixed with water to make a light paste can be very successful in treating wounds. You can substitute the hydrogen peroxide in steps 1 and 2 above with this paste. The only disadvantage to using the paste is that the wound will be stained a dark purple for a couple of days which makes it harder to tell if the wound is healing. Aside from that it is a very effective treatment. Do not use on a wound that is  already healing because it will destroy the fragile new cells.

• Sometimes medicated food can be effective, especially if you have a hard time catching the fish or the fish are too small to inject. The problem with medicated food is that often times the fish that need the medication the most are too sick to eat it.

• By far the most effective way to treat a bacterial infection is to inject the koi with antibiotics. This gets the proper amount of medication directly into the koi. The easiest way to inject a koi is to guide the koi into a clear plastic bread bag, drain out the excess water, roll the koi over on its back, pull the excess bag back until the bag is tight around the koi and then inject the koi through the bag in the area between the anus and the ventral fins at a slight angle toward the head. The koi should be given an injection once a day for three days in a row and then every other day for two more injections. If the water is below 55 degrees spread out the injections to one every other day for a total of 5 injections. This is necessary because the koi’s metabolism is very slow below 55 degrees. To inject the koi, use a 1cc diabetic syringe with a 25 gauge, ¾ inch needle. You will need to get the antibiotics through your local veterinarian.

• Heat is very important in the healing process. During the summer months when the water is warm, bacterial infections will grow very fast, but they will also heal very fast fast once properly treated. During the winter time when the water is below 58 degrees koi fish wounds take a very long time to heal. If you have the ability to slowly warm up (5 degrees per day) the water the koi are in, to 76 degrees, it is impressive how quickly they can heal.

• Salt added to the water of koi with bacterial infections can be beneficial. If the infection has created a hole in the outer skin of the koi, water will begin to enter the koi through osmosis because the fluid in the koi is slightly salty. By raising the salt in the water to around .3% it will stop this proccess and spare the koi’s kidneys the stress of trying to remove all this extra water.

• It is important to try and establish why the koi has a bacterial infection to begin with. If it is a secondary infection caused by some initial trauma (whether it is parasites, water quality or the stress of overcrowding) unless the initial reason for the infection is corrected you will not be able to successfully treat the infection.