Toxic materials around the pond. What is safe?

What chemicals can leach out of pond-side materials? What effects will they have on water quality and the Koi themselves? In what ways can you seal these materials? Can you use the same sealant for them all?

When considering the topic of leaching in and around koi ponds, we could easily become paranoid and take the view that our koi basically do not stand a chance. With the abundance of foreign materials in and around our ponds, it won’t be long until our pond becomes a lethal cocktail of chemicals. Add to that the minute amounts of certain chemicals that are required to produce an adverse effect on koi health – so small in fact that it would require top laboratory equipment to detect it, that we might as well give up now.

A common feature of these chemicals is that they also tend to have an effect on our koi over many months, causing general (and usually imperceptible) declines in health – and rarely the acute, traceable incidents that we would expect from ammonia or nitrite. Furthermore, the chemicals may affect koi in specific tissues that do not exhibit clear and noticeable symptoms – such as fertility or growth. And when you consider how simple and basic the ecology in an artificial koi pond is likely to be, any pollution will be directed completely at our koi, with very few lower organisms (that you would find in a natural mud pond) in place to absorb or metabolise pollutants instead of your valuable koi.

Because of water’s characteristics as the world’s best solvent, compounds and chemicals of all descriptions will readily dissolve and find their way into your pond water. Materials that are either in or around your pond will have a tendency to leach into your water, with the list of chemicals being as long as the number of chemicals in and around our ponds! There are however, a few areas and materials more commonly used around koi ponds that are more likely to leach than others.


Pipework, airline, flexible hose, planting baskets and even some types of filter media are all made of plastic and could potentially leach compounds into your pond. Rigid ABS or PVC pipe is regarded as non-hazardous for koi. But having said that, tests have shown that rigid soil/waste pipe (particularly light brown pipe) will release hazardous chemicals into pond water – so choose rigid pipe carefully.

Flexible PVC used in airline and flexible hoses can pose a real risk through the leaching of plasticisers into the water. Plastics that give off that ‘new car smell’ will more than likely contain plasticizing agents called phthalates. These compounds make PVC flexible and have been shown to cause sex changes in fish (in extreme cases) and have also recently been at the centre of a teething-ring scare, where it was feared that infants were ingesting plasticisers that leached from their teething rings. For pipework that is continually in contact with your pond water, the risk of leaching is high – but equally, the reported and tangible effects on koi seen by koi keepers is unclear.


Most metals will have a tendency to corrode or release ions, more so if the pond is salted. With this in mind, you should keep any galvanised or copper objects well away from your pond as these will eventually release heavy metal ions into your pond. Pay particular attention to galvanised Jubilee clips and heaters/heat exchangers that may contain copper. Most heaters are made from either stainless steel or even titanium – but it’s still worth checking.


You may encounter wood in or around a pond in several areas. Bridges and decking will usually be adjacent to a pond, as will pergola timberwork and even filter covers. Most of us will have experienced handling slimy and slippery timber around our pond. This is caused by fungi and bacteria breaking down the organic wood material – inadvertently releasing soluble compounds at the same time. The majority of wood that we purchase for constructing bridges or decking will have been pressure treated or ‘tanalised’ to deter such fungal attack – thereby extending the life of the timber. The chemical agent used in the tanalising process is usually CCA – which stands for Copper, Chrome and Arsenic! You can now see how tanalising prevents wood from rotting – not very nice,  and certainly not meant for your pond. If allowed to leach into your pond, then the outcome is likely to be unpredictable, but certainly undesirable. In fact it could be argued that it would be better to use untreated wood near a koi pond.

Other materials.

Stone / brickwork: The biggest issue with brickwork is sealing it against leaching the harsh alkaline lime from cement and rendering. Although in the scheme of things calcium hydroxide is not toxic and its impact will be ameliorated by the water, it is still an unwelcome and unplanned additive to a pond.

Fibreglass: Uncured fibreglass can be a source of unwanted chemicals leaching into a pond. Test whether your fibreglass is fully cured and ‘inert’ by rubbing it with an acetone-soaked cloth; you should not be able to rub off any pigment.

In what ways can you seal these materials?

The best way of preventing unwelcome chemicals from leaching into your pond is to keep them well away from your pond by choosing your materials wisely. However, it is not possible to eradicate every risk so what are your sealing options?

Sealing options:

There are a range of different sealing materials available that are safe to use with koi and ponds. Their suitability for different jobs will be determined by how they are applied and how the surface to which they are applied must be prepared.

  1. Polyurethane varnish.

Seals porous materials and easy to apply. Ideal for sealing brickwork and concrete. Applied with a brush and takes several hours to dry. Solvent-based.

  1. Epoxy-based sealants.

Epoxy-based products come in two packs. When the 2 compounds are mixed, they then cured or ‘go off’ to form the inert barrier. They are solvent-free and do not usually require a primer. Be sure only to mix the quantity you need – and mix it well.

  1. Water-soluble sealant

A new generation of water-based sealants now gives koi keepers the option of an easy-to-use sealant that is also easy to clean. Like the previous 2 types of sealant, these are available in most colours and depending on the manufacturer come in either one or two-step formulations.

Boxout 1

Malachite Green – The deliberate addition of a harmful chemical?

To keep the previous discussion of leaching and pollution in context, did you know that each time we treat our koi with malachite green-based products we are adding harmful toxic chemicals to our pond? Malachite’s toxicity is so widely acknowledged now that it can no longer be used to treat fish that are bound for human consumption. By treating koi for a short term problem with malachite green – what are we doing to their long-term health?

Kill blanketweed and string algae.