Keeping your Koi Fish healthy takes careful preparation and planning. There are several major factors to look out for.
Poor Water Quality
Poor water quality is the leading cause of Koi Fish deaths and Koi Fish Ulcers. Just like humans are concerned with the quality of the air that they breathe, fish need that same quality in water. Keeping a steady pH is crucial. Even if you are in the parameters of a healthy pH (6.8 – 8.2), it still shouldn’t change much, as a sudden change can shock the fish, causing severe stress, or possible death (similar to living in extremely clean air, and then moving to a smokey, humid room).
Another major problem is if a Koi Fish is not getting enough oxygen. If oxygen levels decrease, all fish in a pond can potentially die overnight! Keeping an eye on oxygen levels is crucial. Remember, if you add more fish, you are going to need more oxygen. Changing out water is very important, but remember not to change all of the water at once. It is suggested to change between 25% and 33% of a pond at a time. Any more can shock the fish causing stress, or death. If you live in a climate where the pond is going to freeze, you must remember to expose areas of the surface to allow for gas exchange, and prevent toxic buildup. A great device to maintain oxygen is an aeration device, which disturbs the water constantly, similar to a waterfall.
The second leading cause in Koi Fish deaths are parasites. The leading cause of parasite contraction is through other fish. It is a good practice to quarantine a fish for a few weeks before introducing it to your pond. Although this wont ensure that the fish is parasite-free, it is a strong indicator that it is, and if it isn’t, that the amount of parasites on the fish isn’t reproducing fast enough to harm the fish, or it’s environment. When purchasing a fish, it is always good to ask where the fish is from, what treatments they use to keep their fish parasite-free.
Parasites generally congregate on the sides of the fish, top of the head, and along the base of the tail. The two common parasites that are visible without a microscope are anchor worms, and fish lice.
Fish Lice are about a 1/4 inch in length, and are green, translucent, jelly like creatures. They have several sets of legs that are used to attach to the Koi Fish. To treat, you need to purchase Fluke tabs, or ProForm LA. Assuming that your water doens't need Oxygen, pH or other treatments, you should have no problem ridding your fish of Fish Lice.
Anchor Worms are not actually worms, but copepods. The female will attach her head to the fish by using its head, which is shaped like an anchor. The male worm attaches itself to the female, and fertilizes her eggs. After the eggs are released, the worms die off, leaving open wounds on the fish. These wounds can easily turn into infection. There are several treatments including Fluke tabs that will work, but the most effective way to rid your fish of anchor worms is to qurentine them, and manually remove the worms with tweezers.
An unhealthy fish is an unhappy fish. Not providing the proper elements to resemble a natural habitat can cause serious stress on a fish. It is important to have the correct plants. Having plants that offer oxygen, food, and shade are crucial. If a fish doesn’t have proper shade, they will stress due to heat. Also, if the water is cooler, than it can hold more oxygen.
Lack of Sunlight
Another common problem is adding too many plants to your pond. Although fish like to hide from the sun when it is too hot, having a pond that has too many plants can cause your fish to not get enough sunlight. This is an easy problem to spot and fix. If you have a colored fish that is losing color, it is possible that they have a lack of pigment which can be caused by a lack of sunlight. The easy fix, is to remove some of the plants from your pond. Don't remove all plants, as the fish do need a place to escape from sunlight on bright, hot days, but remove some of them.
If a Koi Fish's eyes start to bulge, a number of causes are possible. The reason that a fish's eyes bulge, is due to fluid buildup either behind the eye, or actually in the eye itself. The most common cause is due to bacteria. If a fish is introduced to the pool, it could be related to stress. It is best to quarentine the fish and look for signs of improvement. Another common behavior is lack of appetite. If your fish stop eating, it cold be due to bacteria in the water. The best remedy is to start to change out water. Over time the fish's eyes should start to retract, and appetites should resume. If not treated in time, the fish's eyes could have to be removed.