How to… Understanding Koi pond buffering

Buffering will help your problems with pH? Syd Mitchell explains how buffering your pond could make a difference, with the help of two honeydew melons, a cherry and three oranges…

Understanding pH
pH is a very important water parameter and many Koi keepers have learnt by heart that it is something to do with the balance between hydrogen ions and hydroxyl ions. But how many can truly say that they understand it?

Fortunately, a detailed knowledge of atoms and the forces that bind them together is not necessary in order to understand the basic principles of pH and how to buffer it to keep it stable. You can understand these principles by thinking of the everyday examples with which we are all familiar. If you think about individual atoms in terms of fruit, figure one illustrates perfectly what happens when fish respire and ‘breathe’ carbon dioxide into pond water. Well, almost perfectly. Bear with me a moment longer.

Expressing equations
Two oranges, or 2 O, looks very much like the number 20. So, to avoid confusion, chemists put the numbers after the letter and also make them slightly smaller. So, 2 H would become H2 and 2 O would become O2 and so on. Oh, and H doesn’t really stand for honeydew melon. In chemistry, it stands for hydrogen. O is therefore oxygen and C is carbon.

So, to explain what happens when fish ‘breathe’ carbon dioxide into water, a chemist would express the equation like this: H2O + CO2 = H2CO3. Or, in layman’s terms, water + carbon dioxide = carbonic acid. Nothing has vanished, and nothing has been magically created. The original atoms have merely shuffled themselves from two separate groups into one group, as you can see in figure one.

Now I have shown that chemistry is as simple as counting different types of fruit and shuffling them around, I can answer the question that I am so often asked, which is what is buffering and how does it actually prevent changes in the pH of your pond water?

Carbon power
Basically, chemicals that try to keep the pH of a Koi pond at a particular value are commonly known as buffers. These are stroppy little compounds that like to get their own way. There are a great many of them and each has a preference for a particular pH. When placed in water, they immediately change the pH of that water until it is exactly where they want it. Not only that, but they will selfishly try to keep it there. Sodium bicarbonate is one such chemical; when it is dissolved in water, it immediately tries to adjust the pH in a pond by releasing or absorbing H+ ions.

If you remember that the value of pH is something to do with the balance between H+ ions and OH- ions, it is obvious that anything that either releases H+ ions or mops them up, can alter the pH to its preferred value. When pH reaches 8.4, sodium bicarbonate stops releasing H+ ions or absorbing any, so sodium bicarbonate is called a buffer with a preferred value of 8.4.

By adding bicarbonate to your pond, you are starting this equation in the middle. It can go either to the left, and resist a fall in pH, or it can go to the right, and resist a rise in pH. Below 8.4, bicarbonate absorbs H+ ions and the equation moves to the left, making the water more alkaline. This continues until the pH reaches 8.4 when the process stops. Above 8.4, bicarbonate releases H+ ions and the equation moves to the right, making the water more acidic. This continues until the pH is 8.4, which is when the process stops.

pH problems
Koi will adapt to any value of pH in the range of 7.0 to 8.5, but the value must be stable. It should not vary by more than 0.2 per day. Biological filtration has a tendency to reduce pH levels. The bugs in biological filters also need carbonate in addition to ammonia (or nitrite) as ‘food’ in order to grow and multiply. They remove over 7mg of carbonate for every 1mg of ammonia they convert to nitrate. As they remove carbonate, there will be less of it to help buffer the pH. If it is not continually replaced, there will come a point when pH levels will suddenly drop, which is called a pH crash and is a potentially lethal situation. Keeping the value of Carbonate Hardness (KH) between 90–150ppm is a sure way of buffering the pH against this problem.

Another pH problem that may not be immediately lethal, but which can cause stress and lead to subsequent health deterioration, is a variation in pH that is caused by plants or algae. Algae and plants both absorb CO2 by day, and give off CO2 at night. If there is insufficient buffering, pH levels will rise as they absorb CO2 and then fall again as they release CO2. Again, keeping the KH between 90–150ppm will buffer the pH against these variations.

Feeling buff?
Sodium bicarbonate is an excellent instant buffer. It is cheap, and easily available from supermarkets or grocers. It is ideal to restore a KH value that has been allowed to become too low, or provide a quick remedy for a pH crash. You cannot ‘overdose’ a pond and shift the pH out of the acceptable range; you just need to add it a little at a time so that the pH does not change too quickly – sprinkle it into a convenient filter bay and allow it to dissolve. Small amounts will shift the pH towards 8.4, and a larger amount will shift the pH to 8.4 and will keep it there. Do not add any more than 30g per 1,000l per day.