E: Pond keepers A to Z … Easter to Eye Fluke


Traditionally, Easter is the time that signals the start of the pond season in the UK. It is a very exciting and mouth watering time of the year. Never to be found at the same date from year to year, the length of the pond season is determined by when Easter falls as it usually marks that the last winter frost has been and gone.

Easter vacations and the associated Bank Holiday are the catalysts for existing or any would-be pondkeepers to visit aquatic outlets and look at this year’s new stock.

For a retailer, Easter is the time by which the aquatic retailer must be stocked ready for the marauding masses of aquatic enthusiasts. It is the latest date by which the first pond livestock of the season must be quarantined and the shelves bulging with this year’s pond hardware and consumables.


Literally means ‘heat from outside’ and is a term that can be given to any cold-blooded animals, including fish. In doing so, it is recognising that the temperature of a koi’s environment dictates the metabolism of these aquatic animals. You may not have considered this before, but it is the fact that fish are ectotherms that:

We dig ponds as deep as possible to keep fish body temperatures as stable and warm as possible in the winter.

Koi are most active in summer, feeding and growing actively and enter a quiescent state in winter when temperatures drop.

Koi are more susceptible to disease coming out of winter when their metabolism and immune response is depressed while the opportunistic pathogens are able to get a hold and attack susceptible koi. Consequently, many koi keepers dose as a matter of course in spring.


Koi eggs are something we are only likely to see in late spring, when mature fish have responded to the favourable environmental conditions, causing them to spawn.

Koi do not show a high level of parental care and will flock spawn in a breeding free-for-all where males will nudge the ripe ‘belly’ of the females. Males can often be over zealous with their attentions often leading to bullied females becoming damaged.

On account of them being poor parent, koi will produce many thousands of eggs per spawn in the hope that at least a handful will reach maturity. Koi eggs are tiny translucent spheres that are highly adhesive, sticking to pond walls and any submerged plants. On being released into the water, eggs will absorb up to 3 times their initial volume of water during a process called hydration. Koi eggs must be fertilised by sperm within 60 seconds of being released, any longer than that and the eggs will be infertile.

Spawning media such as frayed rope or wool can be placed into a pond as koi begin to show the initial signs of breeding behaviour. The eggs will be released onto this media, which can be removed and eggs hatched in another pond. As eggs hatch in 4-5 days (depending on water temperature), egg fungus is rarely a problem and low fertility rates of 30% are rarely exceeded in such a pond flock spawn.


Pond pumps, UV clarifiers and pool heaters all require a safe and reliable source of electricity. Electricity as part of an external circuit can be quite risky as it is open to the elements. Such an electrical circuit servicing a koi pond with the inevitable proximity of water can increase the risks of hazards further, so some safety guidelines should be followed.

If in doubt, whatever the question, consult a qualified electrician.

Always fit a residual current circuit breaker (RCCB) that will trip off the supply in the event of an earth leakage. Something costing a few pounds and fitted correctly could save a life.

Ensure that any connections are watertight, ideally using a recognised junction box.

Pond pumps are sold as safe to use in a pond and if installed correctly, all electrically driven pond equipment will be just as safe as any other every-day piece of electrical equipment. However, electricity deserves a lot of respect and short cuts or risks should not be taken as they will have severe causes for you and your pond.


We care for our koi by caring for their environment. A koi’s environment consists of its immediate surroundings, particularly the quality of the water to which they are permanently exposed to. All successful koi keepers either consciously or subconsciously recognise that a quality environment is the key to years of rewarding koi keeping through the emphasis of design and construction centred on filtration, pond shape, size and depth. A koi keeper should aim as close as they can to total environmental control where through good planning and design, random or unstable factors associated with the pond environment can be minimised for the overall long-term health and benefit of the koi.

A koi’s environment is open to so many variables, where ensuring the stability of these factors is a means of providing a total life support system.

Environmental factors that we need to be aware of and be able to manage are:

Efficient handling and removal of solids through settlement or entrapment.

Effective breakdown of ammonia and nitrite.

Adequate aeration and circulation.

Stable water temperature, where changes are gradual.

Suitable stocking density.

High quality nutrition.

Ergasilus (Gill Maggots)

This parasite is called a gill maggot after their preferred area of infestation and the appearance of this pest (and their maggot-like egg sacs).

This is quite a rare parasite in koi ponds but will cause an affected koi severe irritation, evidenced by flicking and flashing and the gill’s reduced ability to absorb oxygen. Infested fish are likely to be seen gasping at the water’s surface.

Ergasilus has quite a simple lifecycle with the eggs being released by the females while they are still attached to the gill, hatching in the pond and developing into further infective juveniles. Interestingly, only the female becomes parasitic.

Control is similar to any crustacean parasite and is most effectively carried out by using extremely hazardous organophosphorous insecticides. See Dipterex (Previous Issue)


Commonly known as pop-eye, the symptoms show the eyes bulging out of the head in an extreme fashion. Quite easily noticed when koi are viewed from above as the eyes clearly protrude from the clear lines of the head and body.

Exopthalmia can be caused by a variety of factors, and with no single obvious treatment, with successful treatment being quite rare.

Its occurrence is more frequent in koi that have recently been imported, where factors of the transportation have lead to stresses or imbalances within the fish’s body.

Eye Fluke

Caused by a parasite whose lifecycle is very complex and involves a life stage in a snail and a bird. This rarely occurs in UK koi ponds through new infestations but is more likely to be imported in fish from koi farms where the complex lifecycle of this parasite has more opportunity to be completed.

Infestation will cause the eye to become cloudy with low levels of infestation going unnoticed. Koi carrying eye fluke show little behavioural changes, as its effects are to impede a fish’s vision rather than cause direct irritation. For this reason, and its complex lifecycle, eye fluke is a rare occurrence in UK koi ponds.

Kill blanketweed and string algae.