Smart Koi get 25% water change daily when they are indoors for winter in above ground pools. Even when they are outdoors for summer, they get weekly 5-10% water change. Frequent water change is key to healthy Koi.

You may wonder why Koi need clean water if they could grow in mud ponds. Indeed, Koi being Carp, are tolerant of poor water quality. They may survive poor water conditions but they hardly grow to be longer than 18”. It is like kids growing up in slums. They can survive but can not have a good life. I have seen Koi that were grown in retention ponds. In spring, the Koi gasped for air because incoming water brought all the winter salt sprayed on roads, rotting  leaves, pesticides from lawns. In summer, when the temperatures soared to 90 degrees most Koi die because they don't get enough oxygen. This is not how one brings up his/her pet. The difference between a pet and a wild animal is that the pet is given care and the wild animal isn't. A pet is “bred” by a human in order to be taken care of but the wild animal is born.

When Koi breeders grow them in mud ponds, the ponds are designed in way that no contaminants get to the pond. Pond bottom has clay liner. Incoming water is clean. While some hobbyists keep their ponds in poor condition, others go to extremes. They have Polishing Filters, Reverse Osmosis Units and other fancy gadgets to keep water looking like glass. The owner of the pond may be happy that his  investment can be viewed by anyone who visits his pond but Koi are not. First of all, Koi do not like to be seen by birds. Second, Koi want to eat zoo-plankton all day. They do not know that their daily feed is enough for them to grow. Their reflexes tell them they haven't eaten enough.

Why? Koi feel they ate enough only when their gut is stretched to the full extent. The food that they find in nature is not nutritious enough. So, their brain judges by volume. To fill their gut, they need to eat all day. In a pond, when they don't find anything to eat, they get stressed that they may die of starvation. We have never fed our Koi more than 0.5% of their body weight. Yet they weigh more than most Koi. That is because they find quite a bit of stuff on their own, on plant roots and pond walls.

Is algae good for Koi?
Yes, algae is food for many of the zoo-plankton that Koi eat. In rivers, each Koi has 100,000 gallons or more. But in a pond, each may have 500 gallons or less. This is the sole reason why water needs to be clean. If you have algae in your pond, algae will host zoo-plankton and Koi will be happy. But algae also competes with Koi for oxygen and nutrients in water. Just like pond plants, algae produces carbon dioxide in the night which brings down pH of water but produces metabolic products and toxins during the day, raising the pH of water. The daily pH swings will stress Koi. So, water must be buffered well. Buffering is done by raising alkalinity or hardness or kH. If the incoming water is soft, you can keep a bag of coral shells under the return inlets. Calcium carbonate present in coral shells raises alkalinity but  is not very soluble. But when water rushes through coral shells, some amount will dissolve and raise the kH of water.

Back to the algae question. Some amount of algae is good. If your pond water can not maintain stable pH, you have to reduce the amount of algae either by placing shade cloth over the pond or or by adding black dye to you water. First method is the best because it kills algae slowly. If the algae is killed at once (as a result of  algaecide or several cloudy days or adding black dye), it depletes oxygen from water, killing your Koi. Before killing algae, change water to reduce algae and reduce nutrients that are feeding the algae. Then use any of these methods. If you use a shade cloth, do daily water changes and vacuum the bottom to remove dead algae. If the dead algae is not removed from the pond, the nutrients from algae cells are released into water, leading to a second and bigger algae bloom. In addition, decomposing bacteria will use up oxygen to breakdown algae.

The suspended algae can also be made to clump and fall to the bottom but water clarifier. These clarifiers can kill Koi if not used carefully. Any time you Koi larger than 16”, do not use clarifiers. Clarifiers are oxygen-hungry. As soon as you dump them into water, they will remove oxygen from water, killing your Koi. The only way to prevent algae from growing is to have a deep pond with waterlilies covering 2/3 of the surface area and a UV installed in the pond circulation system. Waterlilies will shade most of the pond walls and floor and do not die if there are several cloudy days. But lily pad stalks, bog plants also provide food for Koi. Bog plants are capable of getting oxygen from air and are preferred over water hyacinths, water lettuce and other floating plants. Get rid of snails and other parasites that come with plants by washing thoroughly and by submerging plants in 1: 20 Bleach solution for an hour. Place them in a tub containing circulating water for 3 weeks until all parasites die without a host before putting them in the pond.

Fish produce ammonia and excrete it in the form of urine and feces. In addition they also shed mucus, scales, release hormones, chemical signals. They are capable of absorbing calcium from water but not phosphate if the phosphate is of plant origin. Nitrifying bacteria and anaerobic bacteria also use bicarbonate, calcium, oxygen and other minerals from water. Without water change, these minerals will be depleted. Therefore, a small water change must be done every week. How much change one should do is dependent on the size of the pond and the density of its residents. A good measure is loss in alkalinity or bicarbonate ions. If you lose alkalinity, it will affect nitrification. If the nitrates are high, water change is the solution. So, one should measure not only ammonia and nitrites but also alkalinity and nitrates.


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