F: Pond keepers A to Z … Fibreglass to Fungus


Considered to be the ultimate way of finishing of a koi pond, fibreglass offers many advantages as a choice of pond construction. However, applying fibreglass as a topcoat to a pond can be incredibly messy, expensive and hazardous to health with the real risk from fumes and glass-fibre splinters.

The combination of several layers of fibreglass matting being saturated by rock-hard setting resin ensures that the final product offers immense strength and durability. In addition, unlike a pond liner, fibreglass can be laid into the smallest of folds and creases and can even be finished off with a choice of coloured gel coats.

Before fibreglass can be applied, a blockwork or concrete foundation must be laid onto which the fibreglass will be applied. There are no short cuts with fibreglassing and it is probably wisest if left to the professionals. They will know where to get the materials, how much to order, all of the tricks of the trade and thoroughly enjoy coming into contact with the resin and splinters!


A filter is the heart and lungs of a koi pond. It is the key to achieving acceptable water quality which in turn is the route to successful koi keeping.

A filter is required in a typical koi pond to maintain clarity, and to breakdown the toxic wastes excreted by koi. Through careful design and installation, the functioning of an effective filter can be divided into 4 areas. Each area performs a role complementary to the others, which when taken as a whole perform to provide excellent water quality and should be installed in the following order.

1. Mechanical filtration is a means of removing suspended solids from the water body. This can be achieved by either trapping solids in an easily rinsed media, slowing the water velocity in a settlement chamber so that suspended solids drop out or through changing flow characteristics in a design using ‘up and over’ weir boards or a vortex.

By removing solids first, the remaining media in the filter should remain sediment-free to enable it to function effectively. Mechanical filtration is important in that if it is inadequate the overall performance of the filter will be inhibited.

Biological filtration is achieved by providing a large surface area onto which beneficial bacteria can colonise. A range of bacteria break down toxic ammonia that is excreted by fish into still toxic nitrite and then into less harmful nitrates. This living part of the filter will take months (and even years) to fully mature so during initial stocking it must be educated to handle ever-increasing loads of waste. Failure to do so will lead to a decline in water quality, causing stress and even disease.

Chemical filtration is a way of removing undesirable toxins from the water through chemical interactions rather than living, biological interaction. Zeolite is an ammonium adsorbing resin and activated carbon can be used to remove dyes and other chemicals found in pond water from certain koi diets and treatments. A more recent innovation in the koi market that uses a combination of chemical and physical action to remove organic compounds can be seen in foam fractionators or protein skimmers.

Sterilisation is the final of the 4 parts to koi pond filtration and is carried out using a UV unit, installed inline with the pump and filter. Usually termed a UV clarifier, its role is to burn and clump together microscopic algae cells that cause green water. These clumps can then be removed mechanically.


Koi have 7 fins in total. Two sets of paired fins (the pectorals and pelvics) and 3 single fins (anal, dorsal and caudal fin). The fins act to stabilise the fish during locomotion with the tail and to less extent the pectorals generating most of the thrust in koi.

Things to look out for in fins:

Ragged edges, torn fin tissue or protruding fin rays. These symptoms either indicate an isolated case of fin damage (which is rare) or more likely, a case of fin-rot, a bacterial disease that causes the delicate fin tissue to break down.

A quick way of assessing a koi’s health is to look at its dorsal fin and check if it is standing erect. A dorsal fin that is clamped and folded flush to the body is an indication that the fish is ‘off-colour’. In a healthy fish, the dorsal fin should be held up tightly, as a yacht’s sail.

Reddening of the fins can be caused by the inflammation of the tiny capillaries within the fin tissue. These hard-to-see capillaries become inflamed and visible when a fish is stressed or fighting infection. Again, as with the dorsal fin, this is a handy indication as to the koi’s health.

Food Koi food is the main way a koi keeper can contribute to the health, growth and colour of their koi. The quality, specification and performance of artificial fish diets (including koi diets) have progressed significantly over the past 10 years. Advanced formulations and milling technology means that a quality, high performance product can consistently be produced, even though on appearances pond pellets may look completely alien to a koi.

There are diets to suit all koi sizes, water temperatures, budgets and even life-stages. Koi food formulation does not only concern the energy, protein, vitamin and mineral requirements of a fish, but also takes into consideration for the well being of a pond. Unlike the nutrition for many other captive animals, koi food and other fish foods must be formulated to ensure that they do not adversely affect the balance and water quality of the pond environment.

So effective are many koi diets, that there is no reason why the health of a fish should be compromised through inadvertently feeding a deficient diet.


Formalin is a solution of formaldehyde gas, usually sold as a 40% solution and labelled ‘ Formalin – 40′. Although available over the counter on its own, it is usually used with malachite green from a single stock solution.

Formalin is very effective when treating ectoparasites (those living on the exterior of a fish) such as skin and gill flukes, trichodina, chilodinella and whitespot.

Care must be taken when using formalin, as it is an incredibly nasty chemical that is just as risky for the koi keeper as it is for the koi. It is carcinogenic so care must be taken not to inhale or come into contact with this colourless liquid. It should be stored at room temperature to prevent lethal white crystalline precipitate from forming.

Formalin is best used as a long-term bath, adding it to the pond system. A course of treatments is usually required over several days. Keep the treated water well aerated and test the water after a course of dosing as in high doses it can impair the biological performance of a filter.


Fry, resembling wriggling slivers of glass are often the first indication that your koi have spawned. Upon hatching, fry do not resemble fish. They do not have any gills, absorbing their oxygen straight from the water, and instead of having 7 fins, fry have 1, swimming like a tadpole. Koi fry will not eat until they have absorbed their yolk-sac (usually 2-3 days) and even then, are quite adaptive and opportunistic when they come to feeding, browsing on pond sides, plants and other surfaces.

It is rare for fry to occur in low numbers on account of their highly fertile parents, and to ensure the successful rearing of fry in numbers, a separate and dedicated pond is required. A successful fry rearing system is considerably more low-tech than a koi pool.


Fungus is one of the easiest koi health problems to diagnose and treat. Seen as grey/white cotton wool-like tufts growing from any part of a koi’s body, fungus is more common in spring when koi are more susceptible to disease. Although ever present in the hospitable, damp and nutritious environment of a pond, fungal spores will usually only successfully colonise damaged areas of fish tissue. Abrasions from scratching, spawning and netting or exposed tissue after an initial disease incident are all typical pre-requisites for a fungal infection.

When a koi exhibits a fungal infection, we are only seeing the tip of the iceberg as the invasive fungal filaments (hyphae) can penetrate deep into the vulnerable and highly nutritious tissue beneath the skin. Treatment can be carried out in situ by treating the pond as per the instructions with a proprietary brand fungus remedy.

Kill blanketweed and string algae.