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I’ll make it a little easier by using abbreviations and leaving out some unnecessary words: -
H stands for Honeydew melon. O stands for Orange. C stands for Cherry.
2 H and O plus C and 2 O = 2 H and C and 3 O
So far, so good, but what has shuffling fruit around got to do with koi keeping? Bear with me, this example is not as silly as it may seem at first.
Understanding pH
pH is a very important water parameter and many koi keepers have learned 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 everyday examples with which we are all familiar. If individual atoms are thought of as fruit, the example above shows what happens when fish respire and “breathe” Carbon Dioxide into pond water. Well almost. Bear with me a moment longer.
Expressing equations
Two Oranges, 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’s would become H2, and 2 O’s would become O2 and so on.
Also H doesn’t only stand for Honeydew melon. In chemistry it is Hydrogen. O is Oxygen and C is Carbon.
So, to explain what happens when fish “breathe” Carbon Dioxide into water, a Chemist would say: -
If you follow what happens to the fruit below, you will see that nothing has vanished, nothing has been magically created. We have merely shuffled the original atoms from two separate groups into one group.
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H2O
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CO2
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H2CO3
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PLUS
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EQUALS
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(WATER)
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(CARBON DIOXIDE)
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(CARBONIC ACID)
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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, and if you keep in mind that we are doing nothing more complicated than shuffling 2 Honeydew melons a Cherry and 3 Oranges then you can easily understand the answer.
What is buffering and how does it prevent changes in the pH of pond water?
Carbonates will combine with Hydrogen to form different compounds. Dependent on the pH, the proportions of these compounds compared to each other will change.
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H2CO3 <===> HCO3- + H+ <===> CO3-- + 2H++
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Remember, this is not complicated, just count the fruit, and follow how the H’s move back and forth. The symbol <===> is just a Chemist’s way of saying; ”can change into or change back again” and also you can ignore the little + and - signs after the letters, that’s too complicated for now.
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H2CO3
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<===>
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HCO3-
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+
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H+
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<===>
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CO3--
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+
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2H++
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Can become
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Plus
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Can become
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Plus
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Carbonic Acid
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Bicarbonate
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+
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Hydrogen
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Carbonate
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+
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2 Hydrogen
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In the equation above, reading from left to right, as the pH tries to rise, Carbonic Acid will first dissociate, into Bicarbonate plus one H+ ion, or further into Carbonate plus two H+ ions. (Dissociate is just a Chemist’s way of saying, split up). As it dissociates (towards the right), it releases, first one H+ ion, then a second one. Since pH is the ratio between H+ and OH- ions, what is happening here is that as the pH tries to rise, more and more H+ ions are released into the water and reduce the value of the pH back to what it was before it began to rise.
Reading from right to left, if the pH begins to fall, the reverse happens. Carbonate will grab hold of, first one, H+ ion then a second one, as it changes to Bicarbonate then to Carbonic Acid. This reduces the number of H+ ions and so will increase the value of the pH back to what it was before it began to fall.
Chemicals that try to keep the pH at a particular value are called “buffers”. These are stroppy little compounds. They 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 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 it’s preferred value. When the 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 our ponds, we 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 when the process stops.
 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 the pH. 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 7 mg of Carbonate for every 1 mg 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 the pH will suddenly drop. This is called a pH crash and it is a potentially lethal situation. Keeping the value of Carbonate Hardness (KH) between 90 and 125 mg/L 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 the variations in pH that can be caused by plants or algae. These absorb CO2 by day, and give off CO2 at night. If there is insufficient buffering, the pH will rise as they absorb CO2, and it will fall again as they release CO2. Again, keeping the KH between 90 and 125 mg/L will buffer the pH against these variations.
 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. Just add it a little at a time so that the pH does not change too quickly, it can be sprinkled into a convenient filter bay and allowed to dissolve. Small amounts will shift the pH towards 8.4. A larger amount will shift the pH to 8.4 and will keep it there. Except in the case where it is necessary to quickly raise the pH in the case of a pH crash, do not add any more than 100 gm per 1,000 (20 gm per 1,000 litres) per day.
(Click here for more information on dealing with a pH crash)
 There are other ways to increase KH that are frequently referred to as buffers, such as oyster shells, crushed coral/coral sand or even blocks of plaster of Paris. From a purely chemistry point of view, these are not true buffers because they cannot react quickly to alter the pH to a particular value – they dissolve far too slowly. However, they do add to the KH value, which in turn resists a falling pH. In this respect they are a good addition to a filter system. In hard water areas, tap water contains Carbonates, so water changes may be enough to keep the KH above 90 mg/L However, high KH does not suit everyone. Some experienced koi keepers prefer water with low pH, KH and GH in order to improve skin quality or colouration. Not everyone agrees that this is the case, but for those that do, it is a perfectly valid way to keep koi, although keeping water this close to the lower limits means that far more care and attention has to be paid to it to ensure that the water parameters remain stable. This course of action should not to be undertaken lightly. Ensuring that the KH is sufficiently high to buffer the pH against variations is, in my opinion, good pond husbandry for the average koi keeper
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