O: Pond keepers A to Z … Osmoregulation to Oxygen


Osmoregulation is concerned with the koi’s regulation of its internal tissue fluid concentration. Koi, like any other fish are essentially bodies filled with fluid, living in a liquid environment. Consequently, due to the process of osmosis, koi wage a constant battle against being diluted to death.

What is osmosis?

Osmosis is the movement of water from a high water concentration (such as pond water) to an area of comparatively low water concentration (koi tissue) through a semi-permeable membrane. The gills act as a semi-permeable membrane where their structure allows the free movement of small molecules (such as water) in any direction, but restricting the movement of larger molecules.

What does this mean for koi?

Water will uncontrollably and constantly flow from the pond into the koi. This water, if left unregulated would cause the koi tissues to become so dilute as to reduce and eventually halt their function, causing the koi to swell up and eventually die.

Fortunately, the koi carries out a full-time regulatory function to remove the water at the same rate at which it enters the fish’s body. It achieves this through significant kidney activity, excreting large volumes of dilute urine. The koi also undergo a salt balancing act, retrieving salts from the pond water that are lost from the fish via the gills. These regulatory functions are described as osmoregulation.

Practical implications of osmosis and osmoregulation

Many koi keepers choose to slightly salt their pond as a tonic and mild prophylactic to reduce the likelihood of some diseases. This will reduce the rate at which water floods into the koi and will also reduce the regulatory workload on the koi’s kidneys.

In a more extreme example, where koi (or other pond fish) may be suffering from ulcers, it may be advisable to dose the pond with salt for its two-fold beneficial effect.

Acts as an antiseptic for the open tissue.

Reduces the osmotic gradient, thereby reducing the passage of water into the koi through the open ulcer. This reduces the work that the kidneys must perform, reducing the likelihood of the fish experiencing kidney failure – a common cause of death by fish suffering from ulcers.

Oxolinic Acid + Oxytetracycline.

Two antibiotics that have historically been widely used in the treatment of koi suffering from bacterial diseases.

Oxolinic acid comes as a bright white powder which unfortunately is not water-soluble. It can be administered orally by adding it to food (after dissolving it in vegetable oil or preferably in gelatine).

Oxytetracycline is most commonly found in a highly soluble green powder (not to be mistaken with malachite green) and is traditionally given in bath form. This method of administering antibiotic is most undesirable as it is difficult to dispose of such a bath safely without exerting pressure on widespread bacterial populations increasing their likelihood of developing resistance.

Unfortunately, because of their widespread, prolonged and often indiscriminate use, both oxolinic acid and oxytetracycline are now both largely redundant with most pathogenic bacteria resistant to both. They are only available in the UK through a Vet. by prescription and the number of effective antibiotics is reducing, with koi and other pondfish under attack from many variants of multiply-resistant bacteria.


A term given to describe an animal that eats both animal and plant material. Inheriting their feeding habits and digestive physiology from the carp, koi are omnivorous, consuming both plant and animal matter that will easily fit into their mouth not requiring chewing or biting. Koi are naturally very sedentary feeders and will consume a diverse menu of food both live and dead. Equipped with a keen sense of smell and a set of sensory barbels, this aquatic pig uses its snout to locate the pond’s equivalent of truffles concealed in the silt and sediment.

Koi foods are formulated to accommodate the omnivorous palate of this cousin of the carp, containing a blend of ingredients of both animal and plant origin. Fish meal and poultry meal besides other ingredients are included to satisfy the meat element of a koi’s diet whereas wheat, maize and soya are included to balance the diet with vegetable matter. If koi were not omnivorous, but had a carnivorous desire for more flesh then it is quite reasonable to suggest that koi keeping would not exist. It would not be possible to keep koi as intensively as they are in many koi ponds. Requiring a trout-like diet which is high in animal protein and coated with oil it is likely that the water quality in a pond would deteriorate very quickly. Fortunately their diet is more water-quality friendly, enabling them to be fed satisfactorily on a range of tailored dry diets.


Koi have two opercula, one on either side of the head, protecting the delicate contents of the gill chamber. Widely called a gill plate, an operculum has two main functions.

1. Protection.

The operculum is a bony plate which streamlines the leading edge of fish, helping it to cut through the water. Similar in structure to a scale (but much larger), each operculum grows with the size of the fish, producing tiny concentric rings, similar to tree rings. It protects the delicate gill tissue located in each gill chamber.

2. Breathing

The opercula, when used as a pair, are flexed to produce a unidirectional flow of water through the mouth, leaving at the rear opening of the opercula. Each operculum is hinged at the front, enabling it to flare. When this action is used in conjunction with the fish’s mouth opening and closing, the opercula act as a pair of bellows but in reverse, sucking oxygenated water across the gills. As the water passes through the gills, they take up the majority of oxygen and exchange it with carbon dioxide.

When anaesthetising a fish, the operculum can be used as an accurate indicator to ensure that the koi is not overdosed. At no stage of sedation should the opercula stop moving.

Organophosphates A group of highly poisonous nerve toxins developed to treat external parasites. The majority of organophosphorous compounds are used in agriculture in the treatment of livestock (even though this has been declining more recently). They are a very controversial group of chemicals because of the evidence that some may cause a range of serious side-effects in humans who come into contact with them.

They are classed as a marine pollutant as they indiscriminately wipe out aquatic life. Their main use in the aquatic field is in the treatment of farmed salmon infected with sea-lice.

Many Japanese koi farmers use organophosphorous compounds (e.g. masoten) to treat Gyrodactylus, Dactylogyrus, fish lice, anchor worm, leeches and other external parasites. The active ingredient in masoten is also found in Dipterex, a crop treatment against flies and their larvae. Neither of these products are licensed in the UK for use in aquaculture, nevertheless their use is quite widespread in koi keeping – quite a risky (and unlawful) means of medication. Many koi keepers persist with organophosphates because they are extremely effective at treating external parasites. However, they cannot be used with orfe and tench and can ultimately cause long-term health problems in humans. Is it really worth it?

Overfeeding Feeding is the most interactive time koi keepers have with their koi. It is the opportunity to inspect and even ‘pet’ koi, with these impressive fish becoming incredibly tame at feeding time.

Feeding time can also be a tempting time to offer excessive food in an attempt to be ‘kind’ or to encourage koi to grow at a faster rate.

Unfortunately, overfeeding is far easier to fall foul of with fish than any other pet. When a cat or dog has eaten sufficient, it will leave what is left to return to later. If fish are offered too much food then once they have eaten their fill, the remainder will begin to break down and pollute the pond water.

When feeding koi, it is wise to remember that the pond water is also being fed, and just like koi, there is a limit to how much food a pond system can safely receive each day. Excessive feeding will lead to an increase in ammonia and nitrite and a drop in fish health.

Tips for safe feeding.

Try to feed a little and often, removing any excess uneaten food with a net.

Unless specifically feeding tench or other bottom feeding fish, use a floating food that can be easily judged as to whether it has been eaten.

Remember that far more fish are killed by being overfed with the resultant poor water quality than have been through starvation. I have never seen a fish die through starvation.


Oxygen is the major life-sustaining gas, used in respiration to ‘burn’ food to release energy. Oxygen maintains muscles in an aerobic state for efficient locomotion and other activities.

Oxygen can often be overlooked as a limiting factor in koi, with it generally being accepted that as long as the pond fish are not gasping, then the pond is sufficiently oxygenated. This is not always the case and great benefits can often quite easily be achieved through the addition of extra aeration to the pond.

Oxygen is available in the pond in a dissolved form (DO) where its concentration is inversely proportional to the water temperature. For example, in hot summer weather (when koi use more oxygen) there is naturally less oxygen dissolved in the water. Medications and other substances that may be dissolved in the pond water also reduce DO. Dissolved oxygen is also required by all other forms of aquatic life, from filter bacteria through to aquatic plants (which also give out oxygen in the day time). So pond keepers should manage the DO to provide sufficient levels for all of these consumers to maintain a stable and healthy pond environment at all times.

Kill blanketweed and string algae.