Koi Nutrition and Water Quality

Koi Magazine

Nutrition & Water Quality

Rupert Bridges, Tetra

Most koi keepers know that water quality is critical to the health and quality of their fish. However, perhaps fewer appreciate how significantly water quality is affected by the food they are using. In this article we will attempt to highlight the link between nutrition and water quality in koi ponds.

Managing water quality

The majority of koi keepers appreciate the importance of good filtration for the removal of dissolved and solid waste from the pond. However, it’s important to recognise that the main source of this waste is the food being used. The quality and quantity of food being fed will determine how much solid and dissolved waste is introduced to the pond. Logically, it will therefore also determine how much work the filter has to do in order to keep the environment healthy. A good quality food can make it much easier to manage the quality of the water, and is equally as important as good filtration and regular maintenance.

Solid waste

‘Solid waste’ principally includes food that is undigested by fish and excreted as faeces. The digestibility of a diet depends on the quality of ingredients and manufacturing processes used to make it. In general, proteins and oils are well digested by fish, with carbohydrates showing more variation. For example, for carp (Cyprinus carpio) the digestibility of cooked carbohydrate in maize meal is 97%, whereas in cotton-seed meal it is only 39.4% (Hertrampf, Piedad-Pascual, 2000). Although koi are better than most fish at using carbohydrates, high levels can reduce the overall digestibility of the diet.

If a diet is not well digested, not only will the fish receive less nutrition per gram fed, but greater amounts of solid waste will be produced. This has a number of implications for managing water quality:

• The decomposition of solid waste will use up additional oxygen.
• As solid waste is broken down it will release dissolved waste such as ammonia and phosphate.
• Solid waste will clog equipment, reducing its performance and necessitating increased maintenance.
• Certain disease-causing microbes multiply on solid waste, so the potential for a disease outbreak may be increased.

Nitrogen

The part of the diet that is digested by koi can be used for a variety of processes. These include ‘useful’ ones, such as building new muscle, cells for the immune system, hormones etc, or they may be broken down to provide energy. If proteins are used to supply energy then the end result is the production of ammonia.

Proteins are made up of amino acids, which include a nitrogen-containing amino group. When amino acids from body tissues or dietary protein are used for energy, this amino group is removed and converted to ammonium via a process called transdeamination. The ammonia is then excreted from the body, principally across the gills. In a well filtered pond the resultant ammonia will be converted into nitrate, which can then accumulate.

The level of ammonia (and therefore nitrate) production depends on the formulation of the diet being fed. For example, koi require ten essential amino acids. If one of these is lacking, the process of building new body proteins will be hindered and ammonia production will increase. Equally, diets can be formulated to provide koi with alternative sources of energy – oil and carbohydrate. If more energy is derived from these nutrients, a greater proportion of the protein will be used for ‘useful’ processes. However, there is a limit to this capacity, and excess oil or carbohydrate can bring its own issues.

The use of good quality, well-balanced protein is therefore an important part of a good quality food. It will help to reduce ammonia production and therefore limit nitrate accumulation in pond water.

Phosphorus

Another dissolved waste product affected by diet quality is phosphorus. Although not directly harmful to fish, its inorganic salts (phosphates) will encourage excess algae growth. The objective of a well made food is therefore to limit the excretion of phosphate. However, this has to be balanced against the fish’s requirement for phosphate, as it is an extremely important nutrient.

Unfortunately, not all of the phosphorus in dietary ingredients can be digested and absorbed by fish. In fish meal its availability is around 40-60%, and this can be even lower in plant ingredients. Most fish require around 0.5-0.9% available phosphorus (Halver & Hardy, 2002), meaning that levels in the diet must be a little higher in order to account for its low digestibility in some ingredients.

Good quality foods must strike the correct balance between supplying sufficient phosphorus for the fish, whilst at the same time avoiding an excess. This can be achieved by ensuring the level of available phosphorus closely matches the fish’s requirements. This way, no excess is taken into the body which would otherwise be excreted as dissolved inorganic phosphates (easily available to algae). It is also important to ensure that the overall diet is well digested, to avoid excess losses of phosphorus-rich solid waste.

By looking at how food affects waste production, it is clear that the use of a high quality diet is an important part of water quality management. It should be considered alongside what equipment to use, and what maintenance regime to employ, when deciding how best to care for koi.

Box-out 1, How much to feed?

As well as food quality, food quantity is also an important consideration. The more you feed, the more waste is added to the pond. This is why growth foods are more nutrient-rich; so you can feed a similar amount and get better results without too much additional waste. In general, better quality foods can be fed in smaller amounts, as they deliver more nutrition per gram.

Box-out 2, Tips for controlling waste build up

1. Never be tempted to overfeed – the old rule of thumb of feeding only what the fish can eat in a few minutes is a good one.

1. Observe how much filter maintenance you have to do when feeding a particular diet. This will give you a very rough idea of what kind of waste production you are getting.

1. Use diets that have been researched properly, and that are recommended by fellow koi keepers. Never buy on price alone, as cheaper foods may lead to greater water pollution and less nutrition.

1. Switch to a nutrient-rich growth food in the summer. This will help you deliver the extra nutrition needed, without excessive waste production.