Although it determines the acidity or alkalinity of water, strictly speaking we are actually measuring the quantity or ratio of two important molecules; the hydrogen ion (H+) and hydroxyl ion (OH-), which are responsible for acidity and alkalinity respectively

Some substances release H+ or caused H+ to be formed when they dissolve in water, while others release or create H-.

The process of molecules splitting apart to form ions is called ionisation. At any time, water will contain both specied of ions with pH being a sort of balance sheet showing which is the predominate ion.

The pH scale measures the rations of the relative quantities of H+ and H - on a scale of 0 to 14. Very acidic solutions where the H+ predominates are measured as 0 on the scale and very alkaline solutions in which H - predominates are 14 on the scale. At around pH 7, depending on temperature and salinity, the numbers of both species present are equal and therefore the water is neither acidic nor alkaline - it is said to be neutral.

The pH scale is a logarithmic measurement of the concentration of H+, which means that each one unit change in the scale equals a ten-fold increase or decrease. Thus, 8 is 10-times as alkaline as 7, while 9 is 100-times as alkaline. So, these seemingly small changes actually represent major changes in acidity or alkalinity.

In water that contains very little dissolved substances - the addition of very small
amounts of acid or alkaline substances can cause quite dramatic shifts in pH. If such wide swings were to occur in ponds, for various biochemical reasons, koi and other organisms would not survive.

However, such fluctuations are stabilized by the presence of water-hardness causing substances. These molecules and ions act as a 'buffer' and 'mop up' any sudden changes in the H+ and H- ratio. Water hardness and pH are therefore closely related, with pH stability dependent on the buffering capacity of the water. As a general rule, hard water is usually alkaline and well buffered, whereas soft water is usually slightly acidic and poorly buffered

Many compounds added or dissolved in water will affect the pH by adding or creating
additional H+ and H-. Typically cement or concrete will make water more alkaline. By far the biggest influences are plant and animal respiration and plant photosynthesis.

All submerged plants and animals, including algae, are constantly removing dissolved oxygen from the water and excreting carbon dioxide during normal respiration. The release of carbon dioxide has an acidifying effect. In addition to respiration, during daylight hours all plants, which include algae forms, actively photosynthesis. They absorb carbon dioxide from the water and use the sun's energy to convert it to simple organic carbon compounds. As carbon dioxide in solution is slightly acidic, so as the plants remove it, the water becomes more alkaline. The more sunshine and algae - the more alkaline the water will become.

These two processes, respiration and photosynthesis, carry on alongside each other, with photosynthesis being the dominant during the day. Thus during the day, plants have a net alkalising effect. However, during the night, plants stop photosynthesis but normal respiration continues, so now they only remove oxygen from the water and excretes carbon dioxide as part of normal respiration, with a net acidifying effect. In poorly buffered water this can cause significant pH swings.

The other significant factor affecting pH is nitrification, which tends to have a slight tendency to acidify water as well as removing the 'buffering' capacity or hardness of water.

pH can affect your koi.

Koi prefer a range between 7 and 8.5, and levels outside of this range can cause health problems. There are several ways that pH can affect koi health.

High acidity or alkalinity can cause direct physical damage to skin, gills and eyes. Prolonged exposure to sub-lethal pH levels can cause stress, increase mucus production and encourage thickening of the skin or gill epithelia with sometimes-fatal consequences.

Koi also have to maintain their own constant internal pH. Even small fluctuations of blood pH can prove fatal. Extreme external or water pH can influence and affect blood pH, resulting in either acidosis or alkalosis of the blood.

The other consideration is pH swing. Large fluctuations - even though they may still be within the preferred range - are likely to be stressful and damaging to health.

As well as koi, we should bear in mind that nitrifying bacteria in the filter also have a narrow pH range preference between 7.5 and 8.6.

Changes in pH will affect the toxicity of many dissolved compounds. For example, ammonia becomes more toxic as pH increases.

Variances in pH will also exert an effect on some common disease treatments, so it is important to take account of pH (and usually water hardness) when using treatments. For example, chloramine-T is more toxic at low pH, while potassium permanganate is more
dangerous at high pH.