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How feeding koi or goldfish can affect pond water quality

Every environment shares problems if the inputs into the system are out of balance with the rate at which they are removed or broken down. Currently, on a global basis, this is the case with CO2 and the earth's atmosphere and it is also true for every koi pond, where the regular addition of food into the pond environment will unavoidably lead to a change in water quality.

It is helpful to understand the various implications for both koi and koi pond that are being fed regularly, the reasons why particular changes in water quality occur and the measures we can take to reduce them.

The stability of our pond environment is going to be threatened through the activities of feeding at many different stages from the moment the food hits the pond's surface through to its digestion, assimilation and excretion and ultimate collection in the pond and filter.

The impact of feeding on water quality

1. Leaching. Water is the world's best solvent and will readily draw solutes out of food, the moment the two come in to contact. Many water-soluble compounds in foods (most vitamins and minerals) will have a tendency to leach out of the food into the water and longer the food is in the water before being consumed, the more extreme the leaching is likely to be. There is a danger of leaching making foods nutritionally deficient but most food manufacturers recognise this potential problem and ensure that food contains excess levels of water soluble vitamins. However, even a tiny degree of leaching is inevitable at each feed, causing nutrients to accumulate in the pond water, thus altering the water chemistry. Therefore, even through the natural course of feeding, the balance of the pond's chemistry is under threat.

Pre-soaking Food?

With the extent and speed at which leaching occurs, it should really bring in to question the practice adopted by a number of koi keepers, of pre-soaking food in the belief that it aids feeding and digestion. Pre-soaking food will cause most of the water soluble nutrients to dissolve, increasing the speed at which they leach into the pond. Is this good for water quality and koi nutrition?

Ingestion, digestion and assimilation

Even if koi consumed their food the second it was offered, feeding would still be implications for water quality, through their breakdown and utilisation of food.

As koi are cold-blooded and their metabolism (biological tick-over rate) is governed by water temperature, it is logical that they are offered different types of diet at different temperatures.

There is no reason why koi cannot be offered a 'summer' diet all year round, as koi will simply digest and assimilate (take up and utilise) what they require, excreting any surplus. However, feeding a summer growth food all year round will have a detrimental effect on water quality (which in turn will affect koi health) by the levels of food excreted by koi.

What makes a summer food different from an autumn food?

As summer is the warmest period of the year (allegedly) it is the period when koi will utilise their food for both growth and storage of energy for the fallow winter period that is anticipated by their physiology. Consequently, such diets are high protein and high-energy diets, ready to satisfy their increased nutritional demands. Yet if these diets are offered when koi cannot utilise them efficiently, then levels of excretion will be increased having a knock-on effect on water quality. Excess protein in particular is likely to affect water quality.

If protein in the diet is in excess of what koi require, koi will not utilise all of the protein in the diet for growth, but either break it down and burn it for energy or excrete high levels undigested.

Protein is made up of 4 elements, carbon, hydrogen, oxygen and nitrogen. When protein is used as a source or energy, koi utilise the carbon, hydrogen and oxygen element but excrete the nitrogen in the form of ammonia. Consequently, too much protein in the diet is likely to lead to an increase in the levels of ammonia excreted by koi.

In addition, the undigested proteins excreted by koi (along with other organic material) will attract significant bacterial action by oxygen demanding heterotrophic bacteria. Interested in the organic element of the waste, such bacteria will also lead to an additional release of ammonia into the water. When koi are ever subjected to excess food (both quality and quantity) it will have serious implications for water quality, but if your filtration is adequate, will avert any short term toxicity problems (ammonia) but still ultimately lead to a build up of nitrates. The accumulation of excreted products (both solid and soluble) will also lead to further noticeable changes in water quality.

Build up of soluble organics.

The accumulation of soluble organic pollutants either directly from food or indirectly from excretion will cause water to become discoloured, often taking on a yellow-ish tinge.. This may also be accompanied by excessive frothing that is caused by the build up of soluble organic matter, created around waterfalls or venturis where pond water mixes vigorously with air. The presence of a high organic content in the water causes the formation of stable bubbles, that do not readily burst.

Build up of Inorganics.

Besides the accumulation of nitrates after the filter's handling of ammonia, feeding can also cause quite a build up of other inorganic compounds - particularly phosphates and sulphates.

Although not as directly observable as organic compounds that cause discolouration and foaming, phosphates can lead to the proliferation of algae. As most koi ponds are plant-free and fitted with a UVc to kill green water, blanketweed is likely to proliferate in such a nutrient-rich and competition-free environment.

Reducing the Impact.

In the current fight against global warming, we recognise that a proportion of CO2 emissions can be added through careful management and the use of alternative energy sources. The same is true for reducing the impact of feeding on a koi pond where the main objective is maintaining healthy koi and to provide them with a stable and healthy pond environment. Having identified that feeding can cause both short-term and long-term instability to water quality, our koi's health will benefit if we can reduce such instability.

Measures to reduce the impact of feeding on koi pond instability. 1. Feed the most appropriate diet: Ensure that koi are offered the protein and energy content they require and can most adequately utilise. This means feeding lower protein diets at cooler temperatures, moving to a higher protein diet when the water warms up.

2. Feed on a basis of a little and often: If too much food is offered at any one time, nutrients have longer to leach out into the water. Not only does this risk causing fish deficiencies, but will also taint the water colour and lead to the proliferation of algae.

  1. Reconsider pre-soaking food before feeding as this pre-leaches nutrients prior to adding the food to the pond.
  2. The best strategy for creating a stable water quality is not to leave lengthy periods between water changes. This keeps you on top of any build up of food-related compounds and ensures that the addition of freshwater is not too dissimilar (and therefore stressful) to the pond and to koi.
  3. The installation of a protein skimmer can be a useful tool in removing dissolved organic compounds from the pond water. It should not be used in place of regular partial water changes, but may allow you to reduce the frequency of water changes.

In summary, we feed our koi to enhance their health, growth and colouration, but in doing so, unintentionally present our pond with potential water quality problems which must be managed to maintain a stable and healthy pond environment.

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