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The Most Important Part


Oxygen is something that most living things need to survive. All the different kinds of life that live in a koi pond depend on the oxygen in the pond. This includes the fish and the bacteria that live in the filter and break down waste. If the oxygen level drops below a certain level, most living things, like fish and filter bacteria, will be hurt and will eventually die. There are many things that can change the amount of oxygen in water. Some of these are natural changes that happen every day or seasonally, and others are caused by the fishkeeper, like adding chemicals. There are also a lot of things that can indirectly change how much oxygen is in the pond.


Let’s start with how oxygen acts in water.

The amount of dissolved oxygen that water can hold is not very much. On average, 210 cm3 of oxygen can be found in one liter of air. One litre of pond water at 10°C, on the other hand, can only hold 6.9 cm3 of oxygen. So, we can see that there is much less oxygen in water than in the air. This means that aquatic organisms must be very good at getting oxygen from their environment. Gaseous exchange, which takes place at the gills, is an excellent way for fish to get oxygen from the environment and release carbon dioxide. By looking at the amount of oxygen in the blood and comparing it to the amount of oxygen in the environment, we can see that fish are better at using the oxygen they have. The most oxygen that can be found in the blood of a common carp is 8.0% by volume. This means that 86% of the oxygen in the blood is used. In other words, the carp is 86% good at taking oxygen out of the air. Compared to this, the most oxygen that can be found in human blood is 19.6%. But only 34% of the oxygen in the air is used. This means that humans are not very good at taking oxygen out of the air. There are many things that can change how well oxygen dissolves in water. The first is temperature. As the temperature goes up, oxygen dissolves less in water. High temperatures don’t usually cause oxygen shortages in garden ponds in the UK. However, this is a real concern in tropical fish aquariums and for people who keep ponds in warmer places like parts of the USA. The amount of salt in water also has a big effect on how much oxygen is in it. Again, this doesn’t really matter for koi keepers, but it is a very important thing to think about when keeping a marine aquarium. Some koi keepers keep the salt level in their ponds at a low level, which causes a small drop in the amount of oxygen in the pond as a whole. The main things that affect how well oxygen dissolves in water are temperature and salinity, which don’t matter too much for people who keep ponds in temperate climates. But there are a lot of other physical, chemical, and biological things that can have a big impact on the amount of oxygen in the air. Also, koi keepers do a lot of things that can directly or indirectly change the amount of oxygen in the water. This can affect the health and long-term well-being of our koi.


Chemistry and physics

When the air pressure is low, the difference between how much oxygen is in the air and how much is in the water is less steep. So, less force is pushing the oxygen into the water. When the air pressure is low, like when it is stormy, thundery, hot, and humid in the British summer, the oxygen level in the koi pond can drop. Biological All of the living things in the pond need oxygen to stay alive. Fish, filter bacteria, invertebrates, and other small creatures all need a certain amount of dissolved oxygen to live. Just like animals, the plants and algae in the pond need oxygen to live. Dissolved oxygen is always being taken away, so oxygenation at the water’s surface is very important to make up for this. Plants and algae are also a source of oxygen because they use it to grow. During the day, they use the energy from the sun to fix the carbon dioxide in the water. This process is called photosynthesis. As a waste, this process gives off oxygen. Photosynthesis only happens during the day, so plants keep breathing at night just like animals do. So it’s not a good idea to count on oxygenating plants to give your fish oxygen. The pond’s oxygen levels are also affected by the sludge and debris in the pond and filters. The microorganisms that break down this waste need oxygen, and the more sludge there is, the more oxygen is needed. This is called the Biochemical oxygen demand (BOD), and it can be measured by figuring out how much dissolved oxygen has left a sample of pond water after a certain amount of time at a certain temperature. The BOD is a good way to tell if water is dirty or not.


Things that the fishkeeper brought up

The koi keeper’s actions can also have a big effect on how much dissolved oxygen is in the pond. The level may be directly affected by how many air pumps or waterfalls are in the pond and how long they are left running during the day and year. But the person who takes care of the koi can also indirectly change the amount of oxygen in the pond. If you feed the fish too much, their waste will build up, which will raise the BOD. This will also cause nitrate and phosphate to build up, which could lead to algae blooms. Then there might be changes in the amount of oxygen every day. If you don’t take care of your pond, debris will build up, which will raise the BOD. Adding chemicals to the water to kill parasites will also change the amount of oxygen in the water. Formalin is a chemical that shows this. For every 5 ppm of formalin, 1 ppm of free oxygen will be used. So, if the pond already has low oxygen levels and formalin is added, the results could be terrible.


How can a low oxygen level hurt the health of our koi?

A koi pond needs at least 8 parts per million (ppm) of oxygen. Minor, temporary changes below this shouldn’t cause too many problems as long as the pH, Ammonia, and Nitrite levels are good. But a value that stays below 8 ppm will cause health problems for koi. At 5 ppm, koi can live for a few days, but if the oxygen level drops to 3 ppm, the fish will have a hard time getting enough oxygen and will soon die.


So, what are the signs of low oxygen, or anoxia, and how do you know if you have it?

When there isn’t enough oxygen in the water, fish do one thing: they breathe faster. If you spend a lot of time with your fish, you’ll notice that they breathe in a natural rhythm. If there isn’t enough oxygen in the pond, this rhythm will speed up. Also, Koi will swim to the top of the water, where there is more oxygen, and they may hang out near fountains, waterfalls, filter returns, etc., where the water has a little more oxygen. When oxygen levels are low for a long time, fish bodies change in big ways. At first, the fish will make more red blood cells, which carry oxygen through its blood. This makes the blood carry more oxygen, but it uses up a lot of the fish’s energy. Because it takes a lot of oxygen to process food, eating more food doesn’t meet the higher energy needs. So, the fish eat less, and in the long run, they lose weight. So, the metabolism is messed up, which can hurt important organs like the liver. The metabolic disturbance means the fish can no longer support the increased amount of red blood cells, and the total counts begin to drop. This causes anemia in the fish, and the oxygen carrying capacity of the blood falls. Low levels of oxygen also reduce the efficiency of the immune system. Long-term exposure ultimately leads to a decline in plasma protein levels. These proteins are the precursors to the antibodies the fish use to fight the war against parasites. When you add in the fact that many parasites are well-suited to environments with low oxygen, the fish really don’t stand a chance. Remember that oxygen is needed by more than just the koi. The bacteria that live in the filter also need oxygen. If the amount of oxygen in the water drops, the bacteria can’t clean the water as well as ammonia and nitrite. This makes nitrite and then ammonia build up in the water. Both of these are poisonous to fish and make it hard for them to take in oxygen through their gills. So not only are the koi in an area with little oxygen, but their ability to take in oxygen is also being hurt. Oxygen is the glue that holds the pond’s different parts together. If you cut down on the amount of oxygen in the water, the life in the pond will eventually die. Despite how important oxygen is, it is one of the easiest things to keep at a healthy level in water. The amount of dissolved oxygen should always go up when an air pump or venturi is added. Falls and fountains help bring more oxygen into the air. Also, limiting algae growth will help make sure there is enough oxygen at night. Good pond maintenance keeps the amount of sludge and trash to a minimum. This lowers the amount of oxygen that decaying organic matter needs. With an oxygen test kit, you can keep an eye on the amount of dissolved oxygen in the pond at all times. A goal of at least 8 ppm should be made. If the other water quality values are good, a value of 11ppm or more will give you healthy, thriving fish and filters that work very well.


Temporary lack of oxygen? This is a hint:

If you find that the amount of dissolved oxygen in your pond is dangerously low, there is a quick fix to get you out of trouble until the cause of the problem can be found. Just below the surface of the water, a solution of 3% hydrogen peroxide, which can be bought from a chemist, should be squirted into the pond. Carefully pour the solution into a spray bottle, or better yet, use a spray atomiser attachment that fits on the peroxide bottle. Use about 60 sprays for every 86 gallons (388 l) of water in the pond. This is a hard thing to do in big ponds, but it will raise the oxygen levels and give your koi and filters the oxygen they need. It will also give you about 4 hours to raise the oxygen levels in the pond in a more normal way. If you put the chemical on or near the fish, it will hurt their gills. Don’t use this method all the time. If you’re always running out of oxygen, buy an air pump or venturi. The only time you should use hydrogen peroxide is in an emergency.

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