Koi are large and beautiful pond fish, but they can suffer some rather horrible ailments that cause red streaks in the fins and tail. They can result in serious injury or death to the fish. Certain types of gas poisoning can also cause red fins in koi fish. Typically such gas poisoning is the result of having too many large fish in a small pond.
The most commonly seen symptoms of septicemia are sores on the skin, bulging eyes, shredding of the fins, anemia and lack of energy. The fins and tail of a koi afflicted with this disease begin to grow streaky red marks like blood leaking from the veins. These fish's colors fade from anemia. Death can result if this condition goes untreated, but it is an internal infection and not a disease that will spread to other fish in the environment. This disease is also known as red pest.
Tail and Fin Rot
This bacterial disease starts by creating light-colored foggy patches on the fish's tail or fins. This leads to torn and ragged fins or tail as the tissues are slowly eaten away. Eventually the rot eats all of the tissues from the tail or fins, leaving nothing behind but the rays. In the case of tail rot, the disease can progress to the point where it eats not only the tail but begins to chew up the body of the fish as well. Secondary infections can result from this disease, causing the remaining fin tissue to turn red.
Nitrogen in the water -- caused by having too many fish in a limited area creating ammonia from their waste products, which is then converted to nitrites -- can cause koi to turn lethargic to the point that they just sit at the bottom of the pond and barely move, except to scratch themselves against rocks. In some cases the fish might go to the surface and just stay there, not swimming but gasping for air. The nitrites in the water can burn the fins and tails, causing them to turn red.
Too much ammonia results in the growth of too many friendly bacteria that eat the ammonia and convert it to nitrogen. Before these bacteria get established, such as in a new pond, the ammonia levels from fish waste can grow to dangerous levels, causing ammonia poisoning. This condition features symptoms similar to nitrite poisoning but also creates purple or red gills on the fish, loss of appetite and fin clamping. Red fins and tail are also present which eventually lead to tail and fin rot.