pH reflects the acidity or alkalinity of water by expressing the ratio of the hydrogen ion (H+), which is responsible for acidity, and the hydroxyl ion (OH-), which is responsible for alkalinity. Changes in pH greatly influence general water quality and can have a noteable effect on fish health.
Each species of fish has its own comfortable range of pH, with levels outside of this range causing health problems. High acidity or alkalinity can cause direct physical damage to skin, gills and eyes.
Prolonged exposure to sub-optimal pH levels can cause stress, increase mucous production, and encourage epithelial hyperplasia, a thickening of the skin or gill epithelia.
Fish also have to maintain their own constant internal pH. Even small fluctuations of blood pH can prove fatal. Extreme water pH levels can influence and affect blood pH, resulting in either acidosis or alkalosis of the blood.
Changes in pH also affects the toxicity of many dissolved compounds. For example, ammonia becomes more toxic as pH increases.
The pH scale (shown above) measures the ratio of hydrogen to hydroxyl ions on a scale of 0 to 14. The most acidic solutions, are measured as 0 on this scale, while the most alkaline solutions measure 14. At around pH 7, depending on temperature and salinity, hydroxyl and hydrogen ions are in equal proportion and therefore the water is neither acid nor alkaline, but neutral.
The pH scale is logarithmic, which means that each unit change is equal to a ten-fold increase or decrease. Therefore, seemingly small changes in pH actually represent major changes in acidity or alkalinity.
Respiration and Photosynthesis most greatly influence the pH of natural waterbodies.
Aquatic animals constantly remove dissolved oxygen from the water and excrete carbon dioxide through respiration. The release of carbon dioxide has an acidifying effect on water.
During daylight hours all plants actively photosynthesize. Plants absorb carbon dioxide from the water and use the sun’s energy to convert carbon dioxide into simple organic carbon compounds. Asplants remove carbon dioxide, the water becomes more alkaline.
The more sunlight and plant biomass, the more alkaline the water becomes. Photosynthesis is dominant during the day, causing an overall alkaline affect. However, during the night, plants stop photosynthesis while continuing respiration. At night, plants only remove oxygen from the water and excrete carbon dioxide, causing an acidifying affect.
In poorly-buffered water this can cause significant diurnal swings in pH. Even reasonably well-buffered water may have a moderate variation in pH, with more alkaline water in the evening.
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