A: Pond keepers A to Z … Aeration to Argulus


Aeration is vital in the artificially stocked koi pond to maintain sufficient oxygen concentration in the water for all oxygen consumers (fish, bacteria, invertebrates, algae at night etc.). Oxygen can be easily overlooked as a limiting factor of for fish and bio-filters with most ponds and filter systems benefiting from additional aeration, especially at higher water temperatures.

Besides ensuring that there is adequate exchange of gases in the pond, aeration is useful at helping to mix and circulate water within the pond.

Methods of aeration.

Air pumps with diffuser. (Air-domes, air stones, porous pipe etc)

Venturi. Air is injected into pumped water return to the pond.

Waterfalls/fountains cause surface agitation and enhance the diffusion of oxygen from the atmosphere.

Koi require at least 6 mg of oxygen per litre of water. Dissolved oxygen (D.O.) concentrations can be measured using an electronic meter or colormetric test kit. Signs of low DO include lethargic fish activity and in extreme cases, gasping at the water surface.

Aerobic and Anaerobic Conditions

Aerobic means ‘in oxygen’ whereas anaerobic means the opposite. A pond and biological filter should definitely be aerobic environments where dissolved oxygen abounds to be consumed by fish and filter bacteria.

Anaerobic conditions in a pond should be avoided and are associated with black sludgy pond bottoms which produced bubbles of hydrogen sulphide gas smelling like rotten eggs.

However, some filter systems can be specifically designed with an aerobic chamber that will allow anaerobic bacteria to convert nitrates into nitrogen gas, reducing the likelihood of algae problems. An anaerobic chamber will take a slow ‘bleed’ off the main system and will take weeks to fully mature. (see diagram).


A group of pathogenic and non-pathogenic bacteria responsible for causing a number of koi diseases, notably ulcers. Aeromonas bacteria are present in every pond and will strike in fish weakened by stress, abrasions or poor water quality.

These bacteria are enemy No.1 for koi dealers who regularly receive imported koi, stressed from a long journey. Every bit of care is taken to reduce stress and bacterial levels as soon as the koi are unpacked to reduce the likelihood of them breaking out in ulcers. As soon as an ulcer develops on a koi renders that fish un-saleable and is likely to take weeks to fully recover, often losing a patch of scales in the process. Left untreated, open ulcers will eventually lead to the death of a fish. They are best treated using a combination of salt baths and antibacterial/antibiotic treatments.


Algae are primitive plants ranging in size from single cells to massive sea kelp over 50m in length. Algae usually have rapid rates of colonisation and growth causing a ‘bloom’ in ideal conditions, especially in waters high in nitrates and/or phosphates.

There are two kinds of algae affecting koi keeping.

1. Single-celled algae (phytoplankton). The sheer density of these organisms is pond water can turn it pea green. Such algae are the koi farmer’s friend providing an abundant source of food for his stock but conversely are the koi keeper’s foe, reducing the visibility of his/her koi.

Green water can be controlled by using a UV clarifier installed at the correct size for the volume of a pond. It is a guaranteed way of producing crystal clear water that is harmless to fish.

2. Filamentous algae (blanketweed). Algae are present in every koi pond and grow out of control under favourable conditions. Unfortunately as a result of controlling green water, blanketweed is likely to bloom once it is no longer shaded out of sunlight by single-celled algae.

Blanketweed can be controlled by herbicide, nutrient removing compounds, magnets and even devices producing radio waves. Another reason why algae seem to proliferate in koi ponds is that research has shown that algal growth is enhanced in well-aerated water.


Ammonia is a highly toxic, colourless waste product produced by koi from the breakdown of protein to obtain energy. Ammonia is lethal to fish even in very low concentrations and can increase the susceptibility of disease.

Ammonia is largely excreted by the gills and can take two forms in solution depending on the pH of water. The more lethal ‘free ammonia’ (NH3) is abundant in alkaline conditions whereas in acid conditions the ammonia forms the less toxic ammonium ion NH4+. However, any level of ammonia should be avoided at all costs and ammonia levels in pond water can be easily tested using a test kit.

If an ammonia reading is present then it is an indication that there is insufficient biofiltration for the quantity of fish in the pond. Stop feeding, carry out a 30% water change and leave until the filter has broken down all of the ammonia. Only start to feed sparingly when the ammonia reading is zero remembering that it is koi food that is ultimately excreted as ammonia.

Anchor worm. (Lernaea)

The anchor worm is not actually a worm but a crustacean parasite (relative of daphnia). One of the few koi parasites that is visible to the naked eye, females are typically seen trailing their two elongated egg sacs hanging on to the flank of the fish. Lernaea penetrate through the skin and scales, deep into the muscle. The point of attachment is liable to infection and koi may exhibit inflamed or blood red spots on the skin.

Most effectively controlled using an organophosphorous insecticide as a pond treatment and individual adult anchor worms should be removed swiftly with tweezers, dabbing the site of attachment with an antibacterial treatment.


Anaesthetics are drugs capable of causing insensitivity to pain. Anaesthetics used in koi keeping are mixed in water into which the koi is immersed. Koi typically take 1-2 minutes to become fully anaesthetised, enabling them to be removed from the water to be examined or treated. Anaesthetics are very useful when treating ulcers and abrasions topically, injecting fish with hormones or antibiotics and when stripping fish of eggs or sperm.

Veterinarians or experienced and qualified koi keepers should only use anaesthetics as the risks involved are quite great. Over anaesthetising a fish will lead to its death and this is common practice of euthenasing fish or killing a sample of fish painlessly for post-mortem or health-check work. Koi are resuscitated by returning them to a tank of freshwater.

Anaesthetics effective on koi include MS222, benzocaine and phenoxyethanol.

Did You Know? Phenoxyethanol also has antibacterial properties and is often included in bubble baths, lotions and gels as a way of keeping them bacteria-free. No wonder we tend to fall asleep in the bath!


Antibiotics (literally means ‘against life’) are naturally occurring chemicals produced by fungi or bacteria that have an antibacterial effect. Now produced synthetically, their use is controlled through prescription and are only available from a vet. on the production of a diseased fish.

Widespread use of certain antibiotics has lead to an increase in bacterial resistance to these compounds (as seen with oxolinic acid and oxytetracycline which are now ineffective against many bacteria). Over the past few years, one of the major issues when treating ulcers on imported fish is that ulcers regularly do not respond to antibiotic treatment due to bacterial resistance.

Antibiotics can be fed in medicated food, given by injection or in a short-term bath. Care must be taken when disposing of such baths so as not to cause other local bacteria populations to become resistant. Antibiotics are most reliably administered by injection where the dose is given in relation to the weight of the fish.


Argulus, commonly known as the fish louse, like anchor worm is visible by the naked eye. A translucent jelly-like disc-shaped crustacean up to 10 mm in diameter, the fish louse feeds on blood by inserting its mouthparts into the body of the fish.

The first sign of Argulus on koi may be them flashing and scratching against the pond bottom trying to free themselves of these irritating bloodsuckers. Treatment is the same as used against anchor worm (see above). Salt baths and antiparasitic pond treatments are also effective.

Kill blanketweed and string algae.