Clays buffers in a Koi Pond.

When we view a pond full of koi, swimming and thriving in a high quality pond environment, we are in fact witnessing the product and embodiment of thousands of different microscopic processes that will never be seen by our naked eye. Every time a koi eats and digests its food, reactions occur within a koi at the molecular level, leading to the successful digestion, excretion or assimilation of its food. We do not see (or perhaps even appreciate) these micro-bioreactions, but nevertheless they are taking place and, with the evidence clear for all see it in the form of healthy, growing and colourful koi.

The same is true of any number of treatments that we might add to our pond to improve water quality or treat disease. We routinely add compounds or products to our pond to ensure that the water quality remains acceptable to our koi. Two such additives include buffers and clays. When we add them, they immediately interact with our pond water at the molecular (and unseen) level. But what exactly is going on out of view, how do they work and how does the addition of each of these additives affect the well-being of our koi?


Koi benefit significantly from stable water conditions, where water quality parameters such as temperature, dissolved oxygen, ammonia and nitrite should be maintained within desirable levels. This is especially true for pH which must be remain consistently between pH7 and pH9

pH is one of the most widely used and quoted tests and should regularly show a slightly alkaline pH. However, several factors can cause the pH to fluctuate either side of this desirable band with predictable adverse effects on koi health. If pH levels do tend to fluctuate wildly, it is critical for the long-term health of your koi.

It is quite natural for the pH of pond water to experience a relentless downward pressure, potentially forcing it below neutral.

pH is a measure of the relative number of free hydrogen ions (H+) in the water. The more free hydrogen ions, the more acidic the water and the lower the pH. A buffer regulates the number of free hydrogen ions in the water and hence stabilises a pond’s pH.

There are several reasons why a pond will accumulate free hydrogen ions. In a natural pond, that is open to natural influxes of freshwater as well as the natural presence of specific minerals, then the pH will remain relatively stable. In a koi pond however, where stocking rates are naturally high, and there is little exposure to diluting water or natural minerals, then the pH will inevitably start to drop.

The pH in a koi pond is under regular pressure from the by-products of biofiltration. When koi excrete ammonia into the pond, the processing of that toxic compound into nitrite by bacteria will inevitably release free hydrogen ions causing the water to become more acidic.

NH4+ + O2 –> NO2- + 4H+ … Ammonia is converted by bacteria in the presence of oxygen into nitrite, releasing free hydrogen ions into the water

As the processes of biofiltration are taking place constantly (for the health of our koi) then it follows that the filtration process will inevitably release a steady load of free hydrogen ions into our pond. We must address this firstly to provide our koi with a stable pH, but also to keep our pond water alkaline. If a pond is allowed to become acidic, then other parameters become very unstable, increasing the risk of further rapid and extreme swings in water chemistry.

Further pressures on a pond to become acidic.

Respiration, which is also a natural a constant process, produces an additional downward pressure on pH through the formation of carbonic acid in the pond.

CO2 + H2O –> H2CO3 –> H+ + HCO3- … carbon dioxide + water forms carbonic acid. This then dissociates into two ions, including a free hydrogen (H+).

Besides koi, heterotrophic bacteria and all invertebrate and plant life will also respire, releasing CO2 and adding to the acidification of the pond. Unless the negative effects that biofiltration and respiration have on pH are counteracted, our pond’s water chemistry will become increasingly unstable with a knock-on effect for our koi.

Bring on the buffers.

A buffer is the solution to this problem and is best thought of it as a ‘mop’ that soaks up free hydrogen ions. By mopping up free hydrogen ions (and incorporating them into other by-products that lock away the free hydrogen ion) buffers successfully reduce detrimental swings in pH.

What is a buffer?

Any compound or material can be described as a buffer if it behaves in a way as described above. The most common buffer in koi keeping is calcium carbonate, or materials that contain calcium carbonate such as limestone chippings, Cockleshell and even calcified seaweed.

How a buffer works.

The key to seeing how a buffer works is to keep your eye on the free hydrogen ion (which is our foe) when it comes into contact with a buffer. When hydrogen is free and unattached it can cause the pH to drop. But if it is bound to another compound it will no longer present pH problem. Let’s look at how a buffer works.

CO2 + H2O –> H+ + HCO3-

A buffer targets the free hydrogen ion (H+) , in this case the buffer is calcium carbonate (CaCO3).

CaCO3 + H+ + HCO3- –> Ca(HCO3)2

Calcium carbonate (which is insoluble and probably placed in a filter) reacts with the free hydrogen to form a new soluble compound (Ca(HCO3)2) called calcium bicarbonate. You can see that the free hydrogen ion is no longer free, but bound up in the bicarbonate ion, where it can no longer cause the pH to drop. Consequently as long as you have undissolved calcium carbonate in your filter your pond water will be protected against swings in pH.

Clever Plants:

During photosynthesis, plants must take up CO2 from the water so they can manufacture sugars and grow. Photosynthesis can become limited at times in ponds if CO2 levels drop, drastically reducing a plant’s ability to grow.

To overcome this shortage of CO2, some aquatic plants (such as Elodea) are able to remove the CO2 that is bound up in calcium bicarbonate (Ca(HCO3)2) that is dissolved in the pond water.

Ca(HCO3)2 –> CO2 + CaCO3 + H2O

(By removing the CO2 that is bound up in calcium bicarbonate and using it for photosynthesis, the by-products of calcium carbonate and water are released. The calcium carbonate is left as a crusty white deposit on the leafs + stems of the plant.

As with anything that is kept in a pond over time, it soon becomes covered in a slimy bio-film so regular movement and agitation of your calcium carbonate material will help to keep it clean and free to react with free hydrogen ions as they pass through your filter.


It is widely recognised that koi exhibit their best health, colour and vitality when they are kept and reared in a clay pond (often referred to as a mud ponds). In fact mud ponds are a Niigata koi breeders’ key resource for producing the world’s finest koi. These ponds are cut out of clay-rich earth, forming a naturally watertight and mineral-rich base on which to grow koi. The water is highly turbid as koi forage in the muddy substrate, stirring up the ultra-fine clay particles, keeping them in suspension. These ponds are not filtered, but are actually fertilised with manure to make them rich with life food. In fact, on the face of it, you could say that the conditions in which koi are farmed are the complete opposite to those in which they are kept in a koi pond.

We keep koi in highly filtered clear water ponds for our benefit, so that we can see them in all their resplendent glory. But by doing so, we are making a compromise between what we want and what is best for our koi. One way of improving the quality of life for our koi in a koi pond is to add a clay to our pond on a regular basis, bringing a little of the Niigata mud pond conditions back to our own garden pond.

What do clays do?

Basically speaking, clays behave like an ion exchange material, releasing and adsorbing ions in pond water. Their performance in this role depends greatly on the type of clay used (which is a function of its microscopic structure)

Clay particles are microscopic plate like crystals and that are loosely held together in sheets that may form into stacks. The sheets are able to slip and move over each other (which gives clays the silky smooth texture when rubbing a wet clay between your fingers). Clays are formed by the weathering of other materials, eventually being laid down as a new clay deposit. For example, when the hard granite mineral feldspar is weathered, it produces a clay called kaolinite. Whereas weathered volcanic ash will form a montmorillonite clay. These two clays will behave very differently in water as a result of their very different microscopic structures. Montmorillonite proving to be more effective in a koi pond than kaolinite

Because of the greater spacing between the montmorillonite sheets (compared with the kaolinite sheets) montmorillonite clays are able to carry a greater number of ions on their surface and are consequently better at remineralising a koi pond.

Because of its mineral make-up, clay particles are typically negatively charged. This means that they attract positively charged ions which will remain closely attracted to each clay particle to be released into the pond water.

It you can see this in action by carrying out the following simple experiment.

Take a beaker of de ionised water. pH 7, GH 0

Add a remineralising clay

Re-test the pH and GH. You will notice that both the pH and GH will increase. When a clay is added to a pond, it will do this on a larger scale.

When a clay is added to a pond, to be as effective as possible it must remain suspended in the pond for as long as possible. This allows ion exchange to take place for a longer period. As you will see from the Niigata mud ponds, the clay particles are so fine that they do not settle but remain permanently in suspension. The same should be true of a clay the you add to your own pond. If it settles out of your pond too quickly and the water soon returns to its crystal-clear state, then it will not have been as effective as a mud pond. A clay should remain suspended for a lengthy period, and as a result will have an ultra-fine particle size. Ultra-fine clays may be hard to mix into a suspension when preparing them, but they remain suspended for longer periods in a pond.

Clays do not dissolve, so once they can no longer be seen in your pond, their effect on water quality will be minimal.

Why do minerals need replacing in a pond?

Koi and all other aquatic life in your pond and filter will take up and utilise minerals that are dissolved in your pond water. Over time, your pond water will become tired and depleted of specific minerals. These can be easily replaced and replenished by the regular addition of a remineralising clay.

It never ceases to amaze me how complex the reactions are that take place between the time we add a clay or a buffer and later on when we see our koi benefiting from those reactions. Fortunately, when adding either a clay or a buffer, there is no risk associated with overdosing (as there might be with a disease treatment), with the chemistry of our pond water and that of the clays or buffers interacting as required to produce a better, more stable pond environment.

Kill blanketweed and string algae.