The original concept of this series of articles was to begin with the most basic principles of a koi pond explaining everything that can influence achieving good water quality and how that water affects koi in a koi pond. I have tried to do this, one small step at a time, assuming that the reader had no prior knowledge whatsoever and using as little technical language as possible. The linked subjects of dissolved oxygen and how the pond pH affects the efficiency of the way in which oxygen is transported around a fishís body were the topics of last monthís discussion.
The topic this month is slightly more complicated. It will require a basic understanding of water chemistry but this will be explained in easy stages. One or two paragraphs may need to be read twice but perseverance will be rewarded by an understanding of pH and how to control it without expensive chemicals or media.
What is pH?
Whenever that question is asked, the answer usually given is that itís the balance between the H+ ions and OH- ions which leaves more questions than it answered. What are these ions? Where do they come from? And why are they messing around with the water in our koi ponds?
You will be very pleased to know that the answers to the first two are surprisingly easy and that I have no intention of explaining why water has a pH. ďWhat is pH?Ē isnít too difficult to explain but to answer the question ďwhy is pH?Ē would, again, be one of those subjects more suited to a science journal than a koi magazine. However, if you can understand the next paragraph, you will understand nearly everything a koi keeper needs to know about pH. Read it twice if necessary!
What is an ion? Quite simply, it is a broken off piece of a molecule. Hold that thought. We all know that scientists call water H2O and it shouldnít be too hard to understand that this means two atoms of hydrogen and one atom of oxygen all stuck together with a kind of molecular glue. In everyday life, glues arenít always perfect and sometimes objects that have been glued together become unstuck. Exactly the same happens in the world of molecules, water molecules also have a tendency to become unstuck. Why they do this is something I promised not to describe. For the purposes of understanding pH in a koi pond and how to control it, it will be enough to remember that with the water molecule, H2O, a very tiny percentage of the total number of molecules in any volume of water break apart. One of the hydrogen atoms will sometimes break off from the other two atoms and go floating away on its own which means that we have a free-floating H, plus the other H still glued to an O.
Thatís it! If you can grasp that concept, it is almost all a koi keeper needs to remember in order to understand the pH of water except for some tidying up and recognising the fact that sometimes the little H ions donít just float freely on their own indefinitely.
Tidying up the fruit
When a water molecule, H2O, breaks up and one of the hydrogen atoms goes floating away, we donít refer to the result as H and HO. For complicated reasons, scientists prefer to call it H and OH. Nothing has changed; the second part is still an atom of oxygen stuck to an atom of hydrogen, it just makes better sense to chemists to call it OH.
When I do talks about water, invariably complicated water chemistry questions are asked that are better answered with a simple demonstration. I canít show molecules because they are far too small to see so I go equipped with some stage prop plastic fruit. Honeydew melons represent an H (hydrogen atom). Oranges are an O (oxygen atom). In addition there are cherries, pears and bananas; the initial letters of each represents an atom. It not only creates amusement but everyone can see exactly what is happening when atoms (individual pieces of fruit) combine to make molecules (groups of fruit) and what is left if they break apart.
When chemicals combine or react with each other, nothing magically appears or disappears; we still have exactly what we started with, itís just that they are arranged differently. The idea of demonstrating this with familiar everyday objects such as fruit isnít as trivial as it may at first seem. It has successfully been used to explain a great many complicated things about water chemistry to those who have wanted to know but had previously thought they could never understand. If you had trouble visualising what causes water to have a pH, see if this helps:
Water is H2O and the molecules can be represented by two Honeydew melons (H) and an Orange (O) stuck together.
Some of those groups break apart leaving a separate H and another H which is still glued to the O.
To tidy up the chemistry, scientists prefer to refer to the second group as OH rather than HO. This doesnít change anything, itís just a more convenient way to do chemistry
What happen to the Hs that break away?
A common description of pH is that it is the balance between those Hs and OHs and that if there is an equal number of each then the pH is neutral. The description carries on to say that if there are more Hs than OHs, the liquid is an acid, or if there are less Hs than OHs the liquid is alkaline.
In my opinion this is an unnecessarily complicated way of saying that if we start with pure water, even if some of the molecules break apart, it is still pure water. Nothing has changed; there is still the original number of Honeydew melons and Oranges. Most of them are still glued together and a few have broken apart, but apart from that, nothing has changed, they are still all there. The pure water is still pure.
If something adds some extra Hs then the water is no longer pure and the resultant liquid takes on the properties we associate with an acid. If something takes away some of the Hs the liquid takes on the properties associated with an alkali.
There are some chemicals that add Hs to a volume of water and there are other chemicals that take them away. It isnít necessary to understand how they do this and so I will spare you the explanation. However some of these chemicals do both. Bicarbonate is a chemical that has an interesting property that can help koi keepers stabilise the pH of their ponds which is a very important thing for us to be able to do since a stable pH is necessary for haemoglobin to efficiently transport oxygen around a fishís body as was described last month.
Bicarbonate as a buffer
Some chemicals have a preferred pH and either release H ions when the pH is higher than their preferred value until they bring the pH down to that value or, if the pH is lower than they would like, they mop up H ions until they bring the pH up to their preferred value. These chemicals are called buffers. Bicarbonate is a powerful buffer and it has a preferred pH of 8.3 to 8.4. Whatever the pH of the pond when it is added, the bicarbonate will pull it towards 8.3 - 8.4, a little bicarbonate will pull the pH gently, a lot will pull it firmly all the way and hold it there.
When bicarbonate is in water with a high pH it breaks apart and it lets go of its hydrogen atom. The bicarbonate (HCO3) becomes a molecule of carbonate (CO3) with an H floating away into the water. This adds an extra H to the water and anything that adds extra Hs will be making the water more acidic. Obviously, just a single molecule doing this isnít going to make any significant difference to the acidity of the water but, in a few grams of bicarbonate, there are astronomical numbers of molecules and they will all be doing the same thing. The net effect of bicarbonate in high pH water is to release so many of these little Hs that it begins to lower the pH.
How the bicarbonate molecule buffers pH
Bicarbonate has the chemical formula HCO3. Its molecule can be represented by the Honeydew melon (H), the Cherry (C) and three Oranges (O3).
When bicarbonate is in water with a high pH it breaks apart. It lets go of its hydrogen atom and becomes carbonate plus a separate hydrogen (CO3 plus a separate H as shown below). Countless numbers of bicarbonate molecules all simultaneously releasing hydrogen ions into the water, will lower a high pH
When carbonate is in water with a low pH it attracts and holds hydrogen atoms and becomes bicarbonate again as shown below. By removing hydrogen ions from the water, carbonate raises a low pH.
The level of carbonate in water is called its KH which is an abbreviation of the original German expression, Karbonathšrte, which literally means carbonate hardness. One way to measure KH is in units that are called degrees of hardness, e.g. the KH is 6 dKH. In this case, for simplicity, the dKH is usually omitted and it would just be referred to as KH 6 or 6 KH. Another way to measure KH is in mg/L, e.g. 100 mg/L. Sparing you the chemistry explanation, one degree of hardness is almost 17.9 mg/L so KH 6 and 100 mg/L are approximately the same value.
There isnít a precise value of carbonate or KH that is optimum for koi, they will be quite happy with values as low as 1 or as high as 10. Since the main purpose of KH in koi ponds is to ensure a stable pH, there are conditions to be observed with very low values of KH, as will be explained in the part of this series that covers reverse osmosis, and so the normal recommended value for koi keeping is that the KH should be kept between 5 and 7 degrees or 90 mg/L to 125 mg/L.
Biofilter bugs consume KH
Between them, as they convert ammonia to nitrite and then to nitrate, biofilter bugs (nitrosomonas and nitrobacter) consume about 7.2 mg of carbonates for every 1 mg of ammonia converted. Since 1 kg of average koi food after being metabolised by the fish will produce about 40 grams of ammonia, this means that for every 1 kg of food fed, 288 grams of carbonate will be consumed and this must be replaced to keep the pH stable. These are very approximate figures because there are a great many variables but it gives an idea of just how much carbonate biofilters consume.
Normal tap water supplies contain some carbonates and in hard water areas there will be sufficient new carbonates added during normal water changes to supply the needs of the biofilter. In soft water areas the carbonate level in the water supply may be insufficient and so additional carbonate must be added.
A convenient way to add carbonate is to use sodium bicarbonate, which is sold in supermarkets and grocery stores. Sodium bicarbonate has the chemical formula NaHCO3 so it contains only 71.4 % carbonate the rest is harmless sodium and hydrogen. In this form it has the advantage of being non-toxic and wonít increase the pH above 8.4 no matter how much is added although the maximum rate of increase in KH shouldnít exceed one degree or 20 mg/L per day, to avoid pH stress and osmotic shock. To avoid this, the maximum amount that should be added per day is 129 grams per 1,000 gallons, although since it is rarely important to increase the KH at the maximum rate possible, I normally recommend 100 gm per 1,000 gallons per day as an easy to remember amount. This will still increase the KH to the required level but, because less is added at a time, it will just take an extra day or two to achieve that value and has the advantage that the rise in KH is a little easier for koi to adjust to.
Next month I will discuss the ammonia parameter and how it is affected by pH. We just canít get away from the importance of pH can we?