Is a pond’s pH important for fish and plants?

You can’t smell it, taste it, see it or hear it, in fact you can’t even bottle it – So what is it? – pH

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.

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 chemistry.

A water quality that is within the koi’s natural limits (ie what its physiology is adapted to and able to tolerate)

Having achieved a suitable water quality, that it is stable, keeping within a koi’s natural limits, otherwise stress and all of its eventualities would occur.

And of course, this applies to 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. Essentially there are 3 problems that may need to be addressed in a koi pond.

The pH is consistently too low

The pH is consistently too high

The pH is very unstable and prone to wild fluctuations.

One of the most significant influences on the pH of our water is its source. Unusually, when trying to create a pond with a suitable pH, the water companies are the koi keeper’s ally (now it’s not often we hear that!). For obvious practical reasons, water companies want the water that leaves their treatment works to be slightly alkaline so it does not corrode their many miles of pipework. To this end, if required, they will actively add Calcium Hydroxide (CaOH) to the water, making it slightly alkaline and non-corrosive. Consequently, upon filling a pond with tap water, we benefit from the CaOH dissolved in the water. However, as soon as that water has filled a pond, it is likely to start changing in its characteristics through various chemical and biological interactions, some for the better, others for the worse.

Why do koi prefer slightly alkaline water?

Koi, as descendents of carp, are a lowland, still water fish. By the time fresh, clean and relatively pure mountain water has reached their natural habitats, it would have picked up an array of salts and minerals, making the lowland waters alkaline and hard. As their physiology has become adapted to this water chemistry, they must be provided with a similar pH and water quality in ponds or they will become stressed. A koi pond must have a pH between 7 and 9, and ideally between 7.5 and 8.5.

What if pH is less than 7.0 and becomes acidic?

In effect, speaking chemically, this means that there is an abundance of free H+ ions. A number of natural biological processes will put pressure on the pH of a koi pond to become acidic.

a. Biological Filtration. When koi release ammonia (NH3) they are releasing nitrogen and hydrogen ions. When that ammonia is broken down by bacteria into nitrite (NO2) the 3 hydrogen ions are released, causing a drop in pH. As ammonia excretion and biofiltration proceed unabated in every koi pond, there is a relentless downward pressure on pH in every koi pond. If the pH is allowed to drop below 7, then every addition of free hydrogen ions accelerates the drop in pH, leading to a pH crash, causing koi and other aquatic life real stress.

b. Respiration. Plants, fish and bacteria respire constantly taking in oxygen and releasing carbon dioxide. This process can also cause the pH to drop as the carbon dioxide combines with water to form carbonic acid. Excessive plant growth can cause the pH to drop to extremes at night, rising again out of the danger zone in the day as plants utilise the carbon dioxide in photosynthesis.

A drop in pH in a pond to below 7 will lead to dramatic changes in koi health, particularly if they are long term. Colours in koi are likely to fade through the deposition of excess mucus while in extreme cases, fish may even be seen to gasp at the surface. Acidic pond water is also likely to be corrosive to exposed plastic and metal surfaces, causing the pond water to become a toxic cocktail of contaminants.

What if the pH exceeds 9.0?

From experience, an excessively high pH is less extreme on koi than a low pH, but should still be avoided. While a drop in pH is caused quite naturally, excessively alkaline water is likely to have been caused by a pollutant, with cement or builder’s lime the No.1 suspect. (This is the same chemical that the water companies add to their supply, but in much lower concentrations). Koi farmers also use builder’s lime (CaOH) to kill off any residual parasites in a mud pond before stocking – such is its caustic nature.

The symptoms of an excessively alkaline pond will cause koi to secrete a slimy mucous film and also lead to gasping at the surface, very similar in fact to those when koi experience acidic conditions.

It is important that the source of the high pH is located and dealt with accordingly. Limestone rockwork or flagging will not be the cause, but the cement work used to fix and lay the landscaping is likely to be. The addition of a neutralising powder during construction or sealing existing and exposed cement work with a suitable sealant should solve the problem.

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. 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.

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 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.

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’ 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 while surrounding landscaping and brickwork should be investigated if an excessively high pH is experienced.

Do’s and Don’ts

Do: 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.

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

Do: 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.

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.

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

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.