What is pH?
As fish keepers we cannot afford to ignore pH. It has a profound affect on an enormous range of biological and chemical processes. Nitrification, photosynthesis and the toxicity of carbon dioxide and heavy metals are just some of the processes affected by this parameter. So what exactly is pH? pH is, in fact, a measure of the concentration of hydrogen ions. Scientifically speaking it can be expressed as;
Let me explain. The log part means that there are ten times as many hydrogen ions present for every change in scale of one unit. The negative sign shows that the higher the concentration of hydrogen ions the lower the pH scale. In other words there are ten times as many hydrogen ions present at pH 6 than at pH 7.
It is recorded on a scale of 0 – 14 with 7 taken as neutral. Below 7 it is said to be acidic and above 7 it is described as alkaline.
What pH do fish prefer?
pH does vary significantly across different natural habitats. Rivers running through rainforests tend to have acidic water whereas chalk streams tend to be alkaline. Marine systems are far more stable with a pH of 8.3. Freshwater fish have therefore evolved to thrive in a range of pH’s. However in general the effects of pH on most pond fish can be summarised in Table 1 below.
|pH||Effect on Fish|
|4 and below||Death|
|4 – 5||Little/no reproduction|
|4 – 6.5||Poor growth|
|6.5 – 9.0||Optimum range for growth and reproduction|
|9 – 10||Poor growth|
|11 and above||Death|
Table 1 : Effects of pH on Pond Fish
How to Measure pH
pH is relatively easy to measure. There are simple test kits available for the hobbyist as well a a vast range of scientific electronic meters. I would suggest that a meter with an accuracy of 0.05 would be well worth the modest investment. The ability to calibrate an instrument against solutions of known pH is also very reassuring.
When recording pH a point worth bearing in mind is that pH can vary greatly over a 24 hour period. The principal variation is due to the process of photosynthesis and the production of carbon dioxide. For a complete assessment of a pond it would be advisable to analyse pH at sunset and sunrise.
Key chemicals affected by pH
Ammonia is the major waste product of all freshwater aquatic animals. It exists in two forms in fish systems that are in equilibrium with each other, ammonia and ammonium.
It is only the unionised form,NH3, that is toxic. The balance of toxic to non-toxic is determined by pH and temperature. pH has, by far, the greater effect. In fact there is a tenfold increase in toxicity for every rise in the pH scale of one. It is impossible therefore to assess the toxicity of ammonia without also knowing the pH. Follow this link for a better understanding of fish ammonia.
2. Carbon Dioxide
Carbon dioxide is produced from normal fish respiration and, like ammonia, will be released into the pond via the gills. It is present in relatively low concentrations in air but is readily soluble in water. Unlike oxygen and other gases however, carbon dioxide concentrations are directly affected by acid-base reactions and hence pH.
Carbon dioxide is roughly 200 times as soluble as oxygen in water and can undergo a series of reactions. It is present in one of three forms;
- Free carbon Dioxide
- Bicarbonate Ions – HCO3
- Carbonate Ions – CO3
Only the free carbon dioxide is toxic. The balance of each of the three forms listed above is determined mainly by the pH as shown in Table 2 below.
|Free Carbon Dixide.CO2||67.7||17.3||2.0||0.2|
|Bicarbonate Ion. HCO3||32.2||82.7||97.4||94|
|Carbonate Ion. CO32-||0||0.6||5.7||37.5|
Table 2 : The Effect of pH on Carbon Dioxide in Water.
Incidentally, pure water has an acidic pH if in equilibrium with the air. (pH of 5.7 at 25°C.)
It is only the free carbon dioxide that is toxic. In most natural waters levels are below 6 mg/l. Once levels approach 20mg/l bad things can start to happen and generally 10mg/l is accepted as a safe level for most fish. Although carp are believed to be more tolerant levels above 60mg/l quickly become lethal. In salmonids levels of 12 -50 mg/l are likely to cause stress and possible nephrocalcinosis. This is a condition where deposits of calcium carbonate are deposited in the kidneys.
There is tendency for novice fish keepers to try to adjust fish tank pH closer to an ideal by adding chemicals directly to the water. This is rarely effective and should be avoided if at all possible. It is usually the case that fish are perfectly capable of adapting to a different pH given sufficient time. Many aquatic outlets are only too keen to sell a variety of powders and potions in the pursuit of the ‘ideal’ pH.
Sudden changes in pH however, can effect fish health and should always be avoided. A better approach to controlling pH is to understand the role that alkalinity plays in limiting pH variation.
Alkalinity is the capacity of water to neutralise acids without triggering a significant increase in pH. It provides a very useful buffering capacity to help maintain stable conditions. Alkalinity is mainly a measure of the bases, bicarbonates and carbonates that are present. A good understanding of the buffering capacity of a fish tank is therefore an essential requirement for maintaining fish tank pH. If this buffering capacity is exhausted a pH crash can quickly very rapidly, endangering the life of the fish.
Alkalinity is closely linked to, but different from, water hardness. It is very easy to confuse the two. Total alkalinity is a measurement of all the bases in water of which carbonate and bicarbonate are the two major players. It can be thought of as the buffering capacity of a pond. This is the ability to resist, or buffer, pH changes. Alkalinity is also referred to as temporary hardness as it can be removed by boiling.
Total water hardness, on the other hand, is a measure of the divalent cations or ions with a 2+ charge. The two most common cations are calcium and magnesium. Rather confusingly both alkalinity and water hardness can be reported in terms of mg/l calcium carbonate (Total hardness is also the same as ‘general hardness’ or GH.)
The most common base making up the alkalinity is carbonate and total alkalinity is often expressed in terms of mg/l of calcium carbonate. It can also be reported in units of dKH or degrees carbonate hardness. One dKH is equal to 17.9 mg/lof calcium hardness. Different species of fish have different preferences for alkalinity depending upon their natural habit. African cichlids like an alkalinity of 10 – 17dKH whilst South American dwarf cichlids prefer an alkalinity of just 1 – 7dKH.
Although a matter of debate it is generally recommended that a total hardness of 65 – 250 mg/l of calcium carbonate (CaCO3) is ideal for most ponds. (3.6 – 14 dH)
Maintaining fish tank pH
Over time, several factors contribute to the lowering of pH in a fish tank. The very process of nitrification depletes the buffering capacity of the pond and leads to a drop in pH. Generally speaking it is good practice to maintain the buffering capacity of a fish tank. Regular water changes is a simple and effective method and for many, this is all that is required.
For more heavily stocked tanks it may be necessary to add carbonate or bicarbonate ions. Oyster shells have long been a popular and cheap source of carbonate. I tend to favour a more soluble form of calcium carbonate marketed under the name of Siltex. It is a popular additive to natural fisheries and helps reduce silt formation. Note that calcium carbonate will result in an increase in alkalinity and hardness. Another popular technique is to add sodium bicarbonate in the form of baking soda. This will increase alkalinity levels but not hardiness. The important thing to remember with all these techniques is not to rush it. Add them slowly and measure their effectiveness with a pH meter. Add more if necessary. A good starting point would be to add 100g of baking soda per 1000 gallons of fish water each day until the pH and alkalinity levels stabilise.
- pH affects most biological and chemical processes important to fish keeping
- Measure using a calibrated meter
- Minimise pH variations by carrying out regular water changes
- Additional pH control, in more heavily stocked ponds, can be achieved by maintaining alkalinity levels
- Alkalinity levels may be maintained with the regular addition of small amounts of baking soda, Siltex or oyster shells