The Vital Ingredient
Oxygen is an essential element for the majority of organisms. The diverse life inhabiting a koi pond is dependent on oxygen; from the fish themselves, to the bacteria colonising the filter and breaking down waste matter, they are all at the mercy of the oxygen in the pond. If the level of oxygen drops below a specific level most forms of life, including the fish and filter bacteria, will be adversely affected and will eventually die. There are many factors that can influence the concentration of oxygen in water; some are natural fluctuations on a daily or seasonal basis and others are bought about by the actions of the fishkeeper, such as the addition of chemicals. There are also many factors that can indirectly influence the concentration of oxygen in the pond.
First let's look at how oxygen behaves in water
The capacity of water to hold dissolved oxygen is small. One litre of air, under average conditions, holds 210 cm3 of oxygen. However, one litre of pond water at 10° C can hold only 6.9 cm3 of oxygen. So we can see oxygen is far more limited in the aquatic habitat than in the air and consequently aquatic organisms must be extremely efficient at extracting the oxygen from their environment. Fish have evolved an excellent system of gaseous exchange (the removal of oxygen from the environment, and the release of carbon dioxide), which occurs at the gills. We can see that fish are more efficient at utilising the available oxygen by analysing the oxygen concentration of the blood, and comparing it to the concentration of oxygen available in the environment. Common carp blood can have a maximal oxygen content by volume of 8.0%, giving a utilisation figure of 86%. In other words the carp is 86% efficient at removing oxygen from the environment. Compare this with the maximal oxygen content of human blood, which is 19.6%. However, the utilisation figure is a mere 34%, leaving us humans rather inefficient at removing the available oxygen from the air. There are many factors that can influence the solubility of oxygen in water. The first is temperature: the solubility of oxygen in water decreases as the temperature rises. While oxygen shortage due to high temperature in garden ponds in the UK is seldom a problem, it is a real concern in tropical fish aquaria, and to pond keepers in warmer climates such as parts of the USA. The salinity (salt content) of water also greatly affects the oxygen content. Again this is not really relevant to koi keepers, but is a major factor to consider when maintaining a marine aquarium. The low level of salt maintained by some koi keepers causes only a minuscule drop in the overall oxygen concentration in the pond. Temperature and salinity are the main two factors that influence the solubility of oxygen in the water, and these are not that relevant to pond keepers in temperate climates. However there are many other physical, chemical and biological factors that can have a profound effect on the oxygen concentration. Also there are many factors bought about by koi keepers that can directly and indirectly affect the oxygen concentration in the water, and thus affect the health and long term well being of our koi.
Physical and Chemical
When the air pressure is low, then the oxygen concentration gradient of air to water is less steep. Thus, there is less force driving oxygen into the water. Weather conditions where low air pressure is dominant, such as stormy, thundery, hot and humid conditions found at certain stages of the British summer, leave the koi pond susceptible to drops in oxygen concentration. Biological The living organisms in the pond are all dependent on oxygen. The fish, filter bacteria, invertebrate and other microscopic life all need a minimum level of dissolved oxygen to survive. The plants and algae in the pond also require oxygen for the same reason as animals. The dissolved oxygen is constantly being removed, so oxygenation at the water surface is essential to replace this removal. Another source of oxygen replenishment is from the plants and algae themselves. During the daylight hours they photosynthesise, using the energy of the sun to fix carbon dioxide present in the water. This process gives off oxygen as a waste product. Photosynthesis only occurs during the day, so at night the plants continue to respire in the same way as animals. So it is unwise to rely on oxygenating weeds to provide your fish with oxygen. The sludge and detritus in the pond and filters also place a demand on the oxygen levels in the pond. The micro-organisms that break down this waste matter rely on oxygen, and the greater the amount of sludge present, the larger the demand on oxygen levels. This is termed the Biochemical oxygen demand (BOD), and can be measured by calculating the amount of dissolved oxygen that has disappeared from a sample of pond water in a given time at a certain temperature. The BOD is a useful indicator of pollution in water.
Factors bought about by the fishkeeper
The actions of the koi keeper can also have profound effects on the dissolved oxygen profile of the pond. Directly, the level may be influenced by the amount of pond hardware such as air-pumps or waterfalls present, and how long these are left functioning throughout the day and year. However, the koi keeper can also indirectly affect the oxygen levels in the pond. Overfeeding the fish will lead to a build-up of waste matter, thus raising the BOD. This will also lead to a build up of nitrate and phosphate, which may lead to blooms of algae. Then there may be oxygen fluctuations on a daily basis. Poor pond maintenance will lead to a build up of detritus which raises the BOD. The addition of anti-parasite chemicals to the water will also affect the level of oxygen. An example is the chemical formalin. Every 5 ppm of formalin will use 1 ppm of free oxygen. Thus if the pond already has a low level of oxygen ad formalin is added, the consequences could be disastrous.
How can a low concentration of oxygen affect the health of our koi?
The minimum level of oxygen in a koi pond is 8 ppm. Temporary minor fluctuations below this should cause few problems providing other water quality parameters (pH, Ammonia and Nitrite) are acceptable. However a permanent value below 8 ppm is destined to cause koi health problems. Koi will survive for a few days at 5 ppm, but if the oxygen concentration drops to 3 ppm the fish will start suffering severe oxygen shortage, and will soon die.
So, what are the symptoms of oxygen shortage, or anoxia, and how can they be recognised?
The primary response of fish in oxygen depleted water is to raise their ventilation rate. Anyone who spends a lot of time around their fish will notice a natural rhythm to their ventilation; if the oxygen level becomes too low in the pond this ventilation rate will be exaggerated. Also the Koi will rise to the surface of the water, where there is more oxygen, and may hang around fountains, waterfalls, filter returns, etc - again where the concentration of oxygen in the water will be slightly higher. Long term low levels of oxygen have profound effects on the physiology of the fish. Initially the fish will increase the nubmer of red blood cells, which carry the oxygen molecule in its blood. This raises the oxygen carrying capacity of the blood but consumes a lot of the fish's energy. The increased demand for energy is not met by an increased food intake, as processing the food consumes a lot of oxygen. So the fish reduce their food intake, and in the long term lose weight. Thus there is a disturbed metabolism, which can lead to damage to vital 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 anaemia 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. Couple this with the fact that many parasites are well adapted to oxygen poor conditions, and the fish really do not stand a chance in low oxygen conditions. Remember, it is not just the koi that depend on oxygen. The bacteria colonising the filter are also dependent on oxygen; should the oxygen concentration of the water drop, the bacteria cannot remove ammonia and nitrite from the water as efficiently. This leads to an accumulation of nitrite, then ammonia in the water. These both have many toxic effects on the fish, not least hindering the oxygen uptake at the fish's gills! So not only are the koi in an oxygen poor environment, but they are having their oxygen uptake ability hindered too. Oxygen is the keystone holding the separate components of the pond together. Reduce the amount of dissolved oxygen, and the pond life will ultimately fail. However, for all its importance oxygen is one of the easiest water quality parameters to maintain at healthy levels. The addition of an air-pump, and/or venturi should always boost dissolved oxygen levels. Waterfalls and fountains help raise the oxygen level. Also, keeping algal growth to a minimum will help to prevent oxygen shortage at night. Good pond maintenance keeps the sludge and detritus at a low level, thus reducing the demand for oxygen by decaying organic matter. The use of an oxygen test kit will enable you to constantly monitor the level of dissolved oxygen in the pond. A minimum of 8 ppm should be striven for. A value of 11ppm or over will give you healthy thriving fish, and excellently functioning filters providing other water quality values are good.
Temporary oxygen shortage? Here's a tip:
Should you record a dangerously low concentration of dissolved oxygen in your pond there is a temporary solution to get you out of trouble until the cause of the shortage can be found out. A solution of 3% hydrogen peroxide, available from chemists, should be squirted into the pond just under the water surface. Carefully decant the solution into a spray bottle, or preferably use a spray atomiser attachment that will fit onto the peroxide bottle. Use roughly 60 squirts per 86 gallons (388l) of pond water. This is a laborious process in large ponds but will raise the oxygen levels giving your koi and filters vital oxygen, and giving you roughly 4 hours to get the oxygen levels back up by more conventional means. Do not apply the chemical on or near the fish or gill damage will occur. Do not rely on this process regularly. If oxygen shortage is a consistent problem, then buy an air-pump or venturi. Hydrogen peroxide is great for emergencies, and should only be used as such.