The Effect of High pH in a Pond on Fish

The pH of water defines how acidic or basic it is. This term describes the amount of hydroxide (OH-) and hydrogen (H+) ions that are contained in the water. Pond water that has equal concentrations of hydroxide and hydrogen ions is considered to have a neutral pH, with a value of 7.0. An excess of hydroxide ions causes the water to be alkaline. The pH scale is logarithmic and the concentration of ions increases tenfold for each shift in the pH value. Pond fish prefer a neutral pH and can be fatally stressed by high pH values.

  1. Physical Effects

    • A high alkalinity value damages the gills, skin and eyes of pond fish. The ongoing exposure of pond fish to elevated pH levels strips mucus from the body of the fish and causes thickening of the gill epithelia. The loss of the protective mucus coating on pond fish predisposes them to bacterial infections, while compromised gill functioning may lead to death. High pH values are far more dangerous to fry and young pond fish and invariably leads to their death in a very short period.

    Indirect Consequences to Beneficial Bacteria

    • Beneficial bacteria convert the toxic metabolic waste products of pond fish into harmless substances. These nitrifying bacteria, which convert ammonia to nitrate (NO3-), are dependent on a stable pH, in the 7.5 to 8.6 range. Elevated pH values negatively affect the function of these bacteria, which impacts on water quality. If the nitrifying bacteria are compromised, toxic compounds that build up in the water will stress the fish and eventually lead to their death.

    Effect on Disease Treatments

    • Numerous disease conditions in pond fish begin as external infections. These infections can become systemic unless treated and are capable of causing the death of these fish. Potassium permanganate is typically used to control a number of parasitic, fungal and bacterial agents and its use normally negates the need for antibiotics. However, potassium permanganate becomes dangerous at high pH values. This makes it difficult to treat injured fish with this medication, which is normally well suited to effectively heal pond fish.

    Toxicity of Dissolved Compounds

    • An elevated pH value increases the toxicity of other substances. Ammonia is ten times more dangerous to pond fish at a pH of 9.0 than it is at pH 8.0. Ammonia causes damage to the gill tissue and blocks the transfer of oxygen from the gills to the blood. Ammonia also destroys the membranes which produce mucus, which damages internal intestinal surfaces. Pond fish which are suffering from ammonia poisoning are visually sluggish and gasp for air at the water's surface.