How PH level of water affect fishes

Most freshwater fish live within a pH range of 5.5 to 7.5 (African chiclids can go up to 8.4). Since the scale is logarithmic, this range represents a variation of over a 1000 times. Even an apparently small change in pH can affect fish, causing stress or death.

It affects their breathing ability. 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 epithelial hyperplasia (thickening of the skin or gill epithelia) with sometimes-fatal consequences.

There are indirect consequences that can also affect fish. Changes in pH will affect the toxicity of many dissolved compounds. For example, ammonia becomes more toxic as pH increases. Fluctuations in pH, even though they may still be within the preferred range, can be stressful and damaging to fish health. Nitrifying bacteria, essential in the conversion of ammonia to nitrate also have a pH range preference, which is between 7.5 and 8.6. Variations in pH will also have an effect on some disease treatments. Chloramine-T is more toxic at low pH, while potassium permanganate is more dangerous at high pH.