Can Koi be poisoned by alcohol?

Dear Ben,

I am a furious mother who stupidly trusted her 17 year old son to stay at home, and look after the house and koi while my husband and I went on holiday. We got back to find several cigarette ends in the koi pond, to my absolute horror. My husband instantly picked them out, and we confronted our son straight away. He confessed to having a party, and just as I thought things couldn’t get much worse, told us how one of his friends decided to feed our koi beer to ‘see what would happen’. I don’t know how much alcohol was actually poured into the pond (I am told that they didn’t pour any directly in the mouths of the koi), but I would like to know, however much or little, if this is harmful towards the koi.


Mrs Newman – Kent

Kids! – who’d have em? I can detect from the tone of your letter how furious you really are at your son’s antics, and the sense of disbelief that he could have allowed your koi to be put at risk. However, I guess that your fish are none-the-worse off for your son’s partying and that if it hadn’t been for the tell-tale cigarette ends, you may never have guessed that your son’s friends had been trying their hand at producing Koi au vin. Oh how I bet your son regrets not giving your pond one last check for any signs of the party!

Recognising how teenagers would sooner have beer freely flowing down their throats than your filter, I suggest that the quantity of alcohol that would have entered your pond was minimal and when taking into consideration the diluting effects of your whole pond (whose volume you do not mention), then the alcohol would represent near drop-in-the-ocean proportions. However, the incident will naturally lead you to speculate about what effects beer may have had on your pond (and koi) had the volumes been much greater.

I would be more inclined to be concerned about the effects that the added organic loading (rather than the alcohol itself) may have had on the pond’s overall balance and water quality. If your pond had been polluted with an excessive organic loading (in this case dissolved organic compounds in the beer – including the alcohol) it would have been the bacteria in your biofilter that would have rescued you from any potential water quality problem

Most of the time we are quite happy to give our filter bacteria the credit of managing the nitrogen cycle in our ponds, breaking inorganic compounds such as ammonia down into nitrite and then into nitrate – but nothing much else on top of that. This is at the top of every koi keeper’s list when discussing the roles that bacteria play in maintaining a koi pond. This is because ammonia is a direct threat to our koi and we have test kits readily available that can measure how well our filter are coping with the breakdown of ammonia into nitrite. But this is merely the tip of the iceberg when considering the breadth of influence that these backroom boys have in maintaining both our ponds and our koi in tip-top condition. Beer would not have added ammonia directly to your pond, but rather it would have been broken down by heterotrophic bacteria.

Heterotrophic bacteria.

Food items that bacteria breakdown can be divided into 2 groups – those that are organic (i.e. contain carbon and are the product of life – which includes beer!) and those that are inorganic (such as simple compounds containing nitrogen (eg ammonia), sulphur and phosphorous etc). Heterotrophic bacteria are concerned with the first group of compounds and are like you and I in that they will digest these compounds so they can absorb the soluble nutrients. Heterotrophic bacteria have a much greater role to play in a mud pond compared to a koi pond as the environment is typically organically rich. However, they are present in a koi pond and their population will explode if presented with a source of food – such as beer – being poured into your pond in excessive quantities.

Heterotrophic bacteria are often described as mineralising bacteria as they breakdown the complex organic molecules into their simple building blocks (or minerals). These same bacteria will accumulate in a settlement or vortex chamber where there is a high concentration of solid organic matter. They will also break down soluble organic matter within the water column. Heterotrophic bacteria demand a great deal of oxygen from the water, something that is directly proportional to the amount of organic matter available (both solid and dissolved in the water). While this does not pose such a problem in a lightly stocked, balanced and natural clay pond, an intensively stocked koi pond could soon become stressed and DO levels start to decline if the organic loading were to increase. It is advisable to keep dissolved organic carbon and particulate organic matter to a minimum in a koi pond by regular purges of mechanical filters and the installation of a protein skimmer to removed DOC. Although heterotrophic bacteria are very obliging and will soon get to work on available organic matter, this could actually have an overall negative effect on the pond’s oxygen budget and you should keep their workload to a minimum.

When things go wrong.

If for any reason, organic matter is allowed to increase in your pond (through an acute incident such as during your son’s party, or through gradual accumulation), then the resulting population explosion of heterotrophic bacteria would soon lead to other problems. Because bacteria are simple, single-celled organisms, they digest organic matter by releasing their digestive enzymes into the surrounding pond water, absorbing the products of digestion through their cell walls. They will do this while being attached to adjacent hard surfaces close to the organic matter (if not the organic matter itself). Consequently, when there is an abundance of organic matter the pond can soon deteriorate to resemble a stomach and intestine, releasing lots of CO2, methane, ammonia with the enzyme cocktail resulting in an acidifying and oxygen-depleted environment. This change in the pond water cannot be tolerated by higher organisms, including koi and would soon result in koi gasping at the surface. However, this would hardly be the case with such a low loading during your son’s party.

Has your pond become discoloured?

A good example as to the extent of organic loading a pond can withstand can be seen when pond water becomes discoloured – resembling very weak tea in extreme cases. If your pond is tea coloured since the party then your son must have emptied a few kegs into the pond – which I doubt. But what causes a pond to discolour in the normal running of a koi pond?

The culprit for making pond water resemble very weak tea is given an all-embracing term of dissolved organic carbon. (DOC). That is, organic compounds that have some how found their way into the pond, becoming ‘dissolved’ in the water. Often loosely referred to as protein, the agents that discolour pond water are actually more varied and will include sugars, organic acids, pheromones, phenols and persistent hydrocarbons (most of which are found in beer). These compounds discolour the water usually giving it a yellowish tinge, and will accumulate over time, creating both an unsightly phenomenon and a degree of instability to the water quality. Being organic, many of these compounds attract the attentions of heterotrophic bacteria, which in turn utilise oxygen accordingly. The greater the organic load, the greater the bacterial activity and the more likely it is for a pond to risk facing an oxygen debt.

There are however, dissolved organic compounds that resist breakdown by even heterotrophic bacteria and it is these that largely accumulate in a pond to cause the unsightly discolouration, changing the snow-white skin of your kohaku into an off-white yellow. Levels of DOC will tend to accumulate over a period of time in a similar way to nitrates, with a regular partial water change the traditional remedy, diluting the problem away.

This can also be overcome by the installation of a protein skimmer.

Protein skimmers (or foam fractionators) have been used as standard kit in marine aquaria as an aid to filtration and a method of maintaining incredibly stable water quality. As this is also the aim of koi keepers, it seemed reasonable to adapt one for use in freshwater. Protein skimmers perform less efficiently in freshwater as it is less dense than marine water and thus cannot create the same efficient fine mist of tiny bubbles. Nevertheless, its performance in freshwater ponds can be quite staggering with respect to the quantity of DOC that is removed from the pond and the resultant clarity of the pond water.

Protein skimming works by a process called adsorption (not to be confused with absorption) which is the attraction of DOC onto a suitable surface. The process takes advantage of the physical nature of DOC molecules, and something that is all too evident on a foaming pond. DOC molecules cause a foam to form as the molecules which collect at the pond surface cause the pond water to form very stable bubbles. These bubbles remain and form into a foam.

If you had had a protein skimmer in operation on your system at the time of the party, then any beer and associated dissolved organic compounds entering the pond would have been removed very rapidly. The amount of ‘protein’ removed and collected would also have given you an idea as to how much beer had actually entered your pond.

A factor of concentration

It may surprise you to learn that many advanced aquarists who keep far smaller volumes of water than a koi keeper in tip-top condition will actually purposefully add alcohol (usually ethanol) to their aquarium water. This is then used by heterotrophic denitrifying bacteria within an anaerobic nitrate filter, so that when they break it down they are forced to use the oxygen for respiration that is chemically bound within nitrate, reducing nitrates to nitrogen gas in several steps. This reduces the nitrate concentration in the aquarium water. So your son’s activities, under certain isolated anaerobic circumstances in your filter could have actually led to a decline in nitrates – benefiting your pond rather than harming it. (But you don’t want him to find that out – do you?)

Furthermore, an organic compound with quite similar characteristics to alcohol – Phenoxyethanol – is used in low concentrations in aquaria against certain external pathogenic micro organisms with no ill-effects to the fish or filter which soon break it down. Even higher concentrations of Phenoxyethanol are used as a fish anaesthetic – again with no long term side effects to the fish.

So besides your annoyance and complete incredulity at your son’s actions while you were away on holiday, unless huge volumes of beer were dumped into your pond (which I doubt), your koi will not have come to any short or long term harm and your biofilter would have been more than capable of handling the additional organic loading. But if you want to continue to put your son through an extended period of guilt and remorse, I suggest you don’t let him read this article!

Kill blanketweed and string algae.