What has poisoned my koi?

Dear Ben

I have a great 2000 gallon pond and a mixture of koi and orfe in my pond. My filter system is quite a straight forward gravity fed filter, and I have three airstones plus a UV.

My pond is quite heavily shaded by tall trees, hence I have no pergola. The problem I have is that in the autumn a few of my deciduous trees shed their leaves into the pond. I have an apple tree as well and it’s difficult to stop small apples and seeds falling into the pond.

Normally I put netting over the pond to stop the worst of it getting in but then I have to net the pond twice a day to make sure nothing is left in there which may be harmful to the koi.

Could you advise me on whether or not apples, seeds or leaves are going to harmful? Should I be religiously netting everything out or would the koi be okay. I don’t want to risk losing of them by trial and error methods.

Thank you

Mr Flood – Newcastle

Dear Mr Flood,

It sounds, from your brief description, that your pond enjoys a good balance between shade and direct sunlight. You also sound to have a maturely planted garden with plenty of trees which are providing your pond with desirable natural shade.

Your question about the potential toxicity or the harmful side-effects presented by leaves and other ‘natural’ matter to koi and other pond fish is one that is frequently asked. I have not seen as substantiated case of a fish dying in a UK pond as a direct result of specific leaf material falling into a pond, but as you will see, there are good reasons why you should keep leaf matter in autumn under control. As you say your letter, you don’t want to risk losing your fish by trial and error methods. Furthermore, just because something is natural does not mean it cannot be harmful or toxic, just take a look at anthrax!

The best approach with regard to leaf matter and apples is no different from our approach to other organic matter that might enter our pond from our garden; We should keep it to a minimum and do our utmost to keep it from entering our pond.

1. Potential Blockages.

Your bottom drain and gravity-fed system will largely be geared at handling fine particulate waste that is typical of a koi pond. However, just as excessive blanket weed growth can block a bottom drain, or even the underground pipe work, then excessive leaf matter might too cause your pipework to block. Unlike their aquatic counterparts, terrestrial plants do not enjoy the benefit of being supported by the water. Consequently, you will have noticed that the complexity and rigidity of the leaf structure of terrestrial plants is far more substantial than that of the relatively limp aquatic plants. This rigidity and inflexibility makes them even slower to be broken down, and a greater risk of causing blockages in pipework. The same is true for pump-fed systems. The most common cause for pumps becoming blocked (besides blanket weed) is the accumulation of relatively tough terrestrial plant matter that has blown in and become waterlogged in a pond, but so far has resisted natural decay.

2. More work for your filter.

The destination of all organic matter will be your mechanical chamber. The majority of it will be relatively fine, waterlogged material produced by your fish. This is unavoidable and is the primary reason for having a mechanical filter. Add to that other ‘external’ sources of solid organic matter (such as leaf matter) and you increase the risk of overburdening your filter. In itself, leaf matter will not be as organically rich as the food we feed our koi, or the faeces excreted by fish, as it will largely be made up of low-nitrogen-containing compounds such as cellulose. But because of their structure, terrestrial leaves will present more of a physical burden being slow to breakdown, but quick to accumulate and possibly block any mechanical media (should you use any) in your first chamber.

3. Toxic or non-toxic?

Leaves from Poplar, Oak, Yew, Elder, Willow or Laburnum trees are regarded as being highly toxic in their freshly fallen state or as the start to break down in your pond or filter (even though first hand experience of their toxicity in garden ponds is scant). But as you are netting your pond, there should be little risk of your fish or pond being affected. Springtime also presents a problem with the fall of blossom, which can be quite considerable in quantity – especially from flowering cherry trees as well as your own apple tree.

Why fear plants and leaves that enter your pond?

Leaf and other organic matter has been falling into natural water bodies containing fish for as long as fish have existed. So why be concerned? Many different plant species from around the world produce compounds that may prove to be toxic to other species of plants or animals. Furthermore, the earliest medicine chests were finished with preparations whose active ingredients were simply botanical extracts. And to this day we continue to use the chemical attributes bound up within the leaves from many plants (eg tobacco and tea plants).

As plant compounds have been studied further, a phenomenon of applied biology called allelopathy has been discovered which is the ability of a plant to produce and release chemicals in the environment that act on or against other organisms. As our gardens are becoming ever more diverse in their planting, with garden centres supplying us with ever more exotic plants sourced from the far reaches of the globe, the chances of our pond fish being subjected to allelopathic effects will continue to increase.

A good illustration of plant toxicity and allelopathy arises from fish farming practices in south-east Asia. Fishermen around the world have used various plant preparations that have been shown to have piscicidal (fish-killing) effects for catching fish. For example, toxic compounds such as Rotenone (isolated from derris plants) and Juglone (from a species of plant found in Japan called Juglans mandshurica) have been used to catch fish by making their environment toxic to fish. Fish farmers in Thailand have experienced to their cost the piscicidal effects of specific plants in their locality. Prior to flooding paddy-fields for growing fish, they would routinely incorporate the aquatic weed Ammannia bacifera into the soil to utilise its allelopathic effect on other plants, there by reducing weed emergence. They did however, from time to time experience unexplained fish kills, which have since been traced back to their use of A bacifera which was initially used to control weeds prior to the land being flooded. It’s use as a weed killer is now being reviewed by these fish farmers.

The Australian aborigines have poisoned waterholes for many generations using leaves which exude noxious compounds when they are soaked in water. This ritualistic method of fish poisoning uses the leaves of various acacia trees (one is even locally known as the ‘fish poison tree’!). Another popular variety of trees that is also likely to occur in our ornamental planted gardens is the eucalyptus which is also used to make the fish in the water holes ‘drunk’ and float to the surface so they can be easily collected. They also use Melaleuca argenta (a close relative to the tea tree, which ironically has recently been used to treat stubborn bacterial problems in ornamental fish). This shows how the same allelopathic compounds can have either beneficial or toxic effects on the same species, depending on the concentration used.


In conclusion, even though the apple tree and its fruit on not regarded as toxic, for reasons other than toxicity, as described earlier, you should do your utmost to keep all terrestrial leaf matter from entering your pond. And with gardens being planted with even greater imagination (with plants sourced from warmer climates, and now able to survive in our warming climate), there is an increasing risk that an acacia or eucalyptus tree may be lurking somewhere in your own or your neighbours’ back garden, there is an even greater incentive to keep your pond free from all terrestrial plant matter.

Kill blanketweed and string algae.