Pond water quality. Will pH affect my fish and pond?

What is pH? What causes fluctuations? What effect will these have on a Koi pond?

Being aware of the many water parameters which we should test for, such as ammonia, nitrite and nitrate, these are measurements of a tangible chemical substance (ie you could buy a bottle of ammonia or a bag of nitrate fertiliser). pH on the other hand is different. It doesn’t actually exist as a substance in its own right but is a description given to pond water as to how it is likely to interact with its environment. Yet pH is probably one of the most frequently tested and commented on of water parameters

So What is pH?

pH is a measurement of free hydrogen ions (hence the H in pH) and recognises that free hydrogen ions make a substance acidic. The opposite of acidic is alkaline, with the pH being measured on a scale of 0 to 14, where 7 is neutral.

In solution, water (H2O) dissociates into two ions, these are H+ (the hydrogen ion) and OH- (the hydroxyl ion). Ions have a charge and as long as the size of the charges are equal and opposite then they will have the potential of forming a stable molecule (e.g. OH- + H+ = H2O).

pH measures the balance between these two ions which make up water, recognising that an increase in the number of H+ ions will cause the pH to become acidic, while an increase in the number of OH- ions relative to H+ ions will cause a solution to become alkaline. So pH is a matter of balance.

Pure water (something that is virtually impossible to create) would have a pH of 7, where the OH- and H+ are in complete balance. However, we can also achieve this balance in any solution, as long as the substances dissolved also lead to a balance of hydrogen and hydroxyl ions.

For example.

Hydrochloric acid (HCl) dissociates in solution into 2 ions, H+ and Cl- (just by looking at the ions formed, it is possible to identify that this substance will make water acidic). If we were to also add an equal amount of an alkali, e.g. Sodium Hydroxide, (NaOH) which dissociates into Na+ and OH­-, making the OH- and H+ ions balance, we can still achieve a neutral pH even though we do not have a solution of pure water.

As koi keepers, we should aim for two things when managing our pond’s pH.

1. A pH that is within the koi’s natural limits pH 7 – 8.5 (ie what its physiology is adapted to and able to tolerate)

2. Having achieved a suitable pH, that it is stable, keeping within a koi’s natural limits, otherwise stress and all of its eventualities would occur. i.e. At all costs we should avoid a fluctuating pH.

There is a constant tug-of-war occurring in a stocked koi pond where various natural factors conspire to alter the pH with a tendency to cause koi problems.

1. 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.

2. 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.

3. 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 (NH3) 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 if the pond pH fluctuates?

You are only likely to notice daily fluctuations in pH if you go to the trouble of testing the pH throughout the day. While daily fluctuations in pH are quite natural in a lowland lake, concerns should arise if the fluctuations in a koi pond are out of the natural limits of koi (ie below 7 and above 9). The pH will fluctuate under the influence of carbon dioxide and ammonia being broken down if there is insufficient buffer in the pond system to mop up the excess free hydrogen ions.

What is a buffer?

A buffer is a substance that will ‘soak up’ any likely extremes in pH and is your ally in the battle against a fluctuating pH. It is a reactive substance that will react with an excess of hydrogen ions should there be any and also release them back again should the pH rise too much.

The most widely used buffer is calcium carbonate (CaCO3) which will prevent your pond from becoming too acidic and reduce the extremities of a fluctuating pH. You can source CaCO3 very cheaply in the form of limestone chips or gravel from a builders’ merchants.

How a buffer works

Let’s say, through extreme levels of respiration, carbon dioxide is released into the water. This combines with water, to form carbonic acid, which dissociates into 2 different ions.
Eg. H2O + CO2            H2CO3        H+ + H+ + CO32-

The free (i.e. unattached) hydrogen ions have a potential to make the water acidic.

Bring on the buffer – Calcium carbonate (CaCO3) to ‘soak up’ those free hydrogen ions, thereby preventing the pH dropping (well, it will actually be in your filter in a mesh bag.

By adding limestone chips to your filter, the following will occur.
CaCO3 + H2CO3         Ca(HCO3)2          Ca++ + HCO3- + HCO3-
Limestone + carbonic acid           Calcium bicarbonate        Which dissociates into Calcium and bicarbonate ions (The hydrogen ions are no longer ‘free’, but attached to other elements, so do not make the pond water acidic).

This buffer will also work in reverse, releasing hydrogen ions back into the water should the water become excessively alkaline. For example, when the pH rises towards pH9, the calcium bicarbonate (which is now dissolved in the water) will release its hydrogen ions, causing the pH to drop slightly (but still remain alkaline).

In summary, Unless you are very vigilant, wild and stressful fluctuations in pH may be occurring undetected, contributing as a stressor to koi health. The addition of a buffer will guard against your water turning acidic or suffering from rapid fluctuations in pH.

BOXOUT: Top tips for managing the pH level of a Koi pond effectively

1. Test your pond through out a 24 hour period one day in the summer to satisfy yourself that your pond pH does not fluctuate to extremes.

2. Add a source of calcium carbonate into your filter as a buffer.

3. Test your tapwater to establish its pH. This will give you a good indication of your pond water’s starting point. Testing for KH will also show you how well buffered it is.

4. Don’t treat your fish immediately upon seeing them secrete excessive mucus. They may not be suffering from a parasite, but merely reacting to a hostile pH. Test the water and/or take a skin scrape.

5. Don’t over react to a single pH reading. Your reading will reflect the time of day, so test 12 hours later for comparison.

6. Don’t respond to an extreme pH by adding rectifying chemical additives to the water. This may result in further rapid fluctuations. If the pH is too high, address the cause and carry out a partial water change. If the pH is too low, add a calcium carbonate buffer and leave it to work.

Kill blanketweed and string algae.