My pond’s pH is too low and unstable.

I became a koi keeper by accident two years ago when I bought a house, complete with koi pond.

It is approximately 15 ft long by 8 ft wide and by my reckoning holds about 2500 gallons. It seems quite easy to maintain, especially as it tops itself up from an overflow that runs off a rainwater butt.

The koi are very tame and range in size from about 8″ up to 18″. As a Father’s Day present my daughter bought me a testing kit for the pond. It tests for ammonia, nitrite, nitrate and pH.

Having read the instructions, my ammonia, nitrite and nitrate readings appear fine, but my pH readings can range anything between 6.5 and 8.0. I have also noticed that my pH changes depending on the time of day that I check it.

The guide that came with the test kit says my pH should be stable between 7.0 and 9.0. Why is the pH so important and what is causing my pH to be unstable, occasionally falling below the desirable range? Is there anything I can do to make my pH more acceptable?

It sounds as though you have inherited and good-sized pond full of healthy koi, and having hit the ground running are keeping your koi in good condition. As you mention, most of your parameters have been confirmed as satisfactory, apart from pH which seems to be fluctuating below the desirable band.

pH is the measure of your pond water’s acidity or alkalinity and as pondkeepers

We are interested in measuring pH for a number of reasons.

The pond’s pH must reflect that of your koi’s natural habitat.

The pond’s pH can actually point to a number of husbandry issues that may need addressing.

The pond’s pH will determine the performance of many important processes within your pond.

Koi require the pH of their environment to fall within the range of 7.0 and 9.0, and preferably towards the safety of the mid-range between these two readings. The pH must certainly not fall below 7.0 to become acidic. The koi’s physiology is geared to work most effectively in the slow, warmer, sediment-rich waters of a lowland plain; an environment that by virtue of the mineral load, will always prove to be alkaline. Even so, koi will still tolerate swings in pH that are quite natural in a mud pond or lowland lake, being brought about by natural bioprocesses within the water body. What they won’t tolerate are deviations in pH outside their natural limits., such as is happening occasionally in your own pond.

What is pH?

pH is the measurement of the acid-forming ‘free’ hydrogen ions in the water. The more free hydrogen ions, the more acidic the water. The pH scale ranges from 0 to 14 with 7 being neutral, above 7 being alkaline and below 7 being acid. At a pH of 7, the number of hydrogen ions (H+) are in balance with the number of hydroxyl ions (OH-) (the two components of water – H2O). A buffer mops up the ‘free’ hydrogen ions, preventing the water from becoming too acidic.

Why does the pH fluctuate in my pond?

There are several factors that can cause the pH of your pond to fluctuate.

Photosynthesis. All plant life in a pond (including uninvited blanket weed) will photosynthesise, using the sun’s energy to convert inorganic carbon (gained from CO2 dissolved in the water) into organic carbon-containing molecules such as sugar. When CO2 dissolves in water, it forms a mild acid called carbonic acid. In the daytime, as the plants photosynthesise, and absorb CO2 from the pond, the level of carbonic acid in the pond drops, leading to an increase in pH.

Respiration. Respiration is the opposite to photosynthesis and results in the release of CO2 into the pond water. Unlike photosynthesis, all aquatic organisms in the pond (both plant and animal) respire and contribute to the CO2 levels in the pond doing so on a continuous basis, day and night. Consequently, the net impact of respiration on a pond’s pH is to make it more acidic, especially at night when it is not balanced by the demands of photosynthesis.

Biofiltration (Nitrification). If photosynthesis and respiration rates are in balance, then even though the pond experiences daily fluctuations in pH, there would still not be a net overall change in pond pH over weeks or months. This is not true for nitrification, whose overall effect is to drive the pH down, making the pond more acidic. The main culprit for starting this chain reaction is ammonia – NH3.

When ammonia is broken down by nitrifying bacteria into nitrite (NO2), the bacteria remove the hydrogen atoms and replace them with two oxygen atoms. These hydrogen atoms become ‘free’ and enter the pond water, causing the pH to fall. As ammonia is constantly being released into the pond, and being broken down into nitrite, then there will be an incessant downward pressure on every pond to become acidic – Something that we must resist on behalf of our koi.

These three natural bioprocesses will combine in every pond to have a potential negative impact on pH. Nitrification causes a chronic downward pressure on pH, coupled with a daily acute fluctuation caused by the interactions of photosynthesis and respiration.

What causes the swings in pH in a natural pond?

A carp’s natural pond environment can be described as eutrophic, which means that it is relatively nutrient-rich and enjoys regular algal blooms (green water) as a consequence. Under these conditions, the whole pond in effect becomes alive and has a real potential for photosynthesis in the daylight hours. In addition, all of the aquatic organisms (including the plants) are also respiring constantly, day and night, and it is these interactions that have an overall impact on the pH of the pond.

In daylight, during photosynthesis, there is a net removal of carbon dioxide from the pond, causing the pH to rise, whereas at night, due to the lack of photosynthesis, the CO2 released by the pond’s respiring organisms is allowed to accumulate, having a downward, acidifying in effect on the pond’s pH.

Why aren’t all ponds and lakes acidic then?

Acids can’t help but react with certain other compounds, many of which are found in most ponds and lakes. These compounds are called buffers, and act like a sponge, soaking up the acid-forming free hydrogen ions, preventing the water body from becoming acidic. The most abundant buffer is calcium carbonate. It is available in many forms including limestone chippings, oyster shell or tufa rock. Calcium carbonate works in same way as chalk-based ant-acid tablets that we might take for indigestion. Acids in the pond react with calcium carbonate on an ongoing basis, forming soluble products that have mopped up the free hydrogen ions, resisting a drop in pH.

Your pond’s pH.

Reading your letter, it sounds as though your pond is experiencing a long-term downward pressure on pH caused by biofiltration, as well as the daily fluctuations caused by the interactions between photosynthesis and respiration. In a well buffered pond, your water would not be able to drop below pH 7.0 and as your recorded pH ranges from slightly acid up to 8.0, I suggest that there is a need for a buffer in your system. This can easily be achieved by adding a mesh bag containing a source of calcium carbonate in your filter. This will help to keep your pH in the more desirable and slightly alkaline zone.

Your low pH also suggests that you may need to carry out a partial water change, but with treated tap water rather than rainwater from your water butt. Even though your ready and regular supply of rain water will keep nitrates under control, it is free of any beneficial minerals that might act as a buffer. Tap water however, is treated by the water companies to be slightly alkaline (so it does not corrode their pipework) and hence offers a buffering action for koi keepers. Rather than completely relying on your source of rainwater for water changes, I suggest you periodically carry out partial water changes with treated tap water.

Something else that can be gleaned from your letter is that you have quite a lot of plant growth in your pond. It is this that causes your daily fluctuations in pH (for reasons described earlier). These daily fluctuations are natural and unavoidable and once you have buffered your whole pond, will not present a problem to your koi, as they will be fluctuating above 7.0, in the range that is natural to your koi.

It is the interaction between photosynthesis and respiration that causes your pH to fluctuate throughout the day, rising in the day and falling at night. This is what you have noticed when testing your pH at different times throughout the day. This highlights that you should measure up your pH at the same time each day to be sure that you readings are comparable. Only then will you know for sure that you have at last gained full control over your pH.

Kill blanketweed and string algae.