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The pH value or "acidity" indicates how acidic or basic (alkaline) there is in water.
It is a dimension for the concentration of acidic or basic components in an aqueous solution.
Pure water reacts neutrally and has a pH value of 7. If acidic components predominate then the pH value drops below 7.
If the basic components predominate then the pH value climbs above 7.

Total hardness is the quantity of magnesium and calcium salts dissolved in the water.
If the quantity of these salts is low the condition is referred to as soft water, if the quantity is high the condition is referred to as hard water.
The specification is made in degrees of German hardness dH; 1° dH equals 10 mg/l dissolved calcium oxide.

Carbonate hardness is the proportion of hydrogen carbonate ions in the pond water and is closely linked with the pH value. The hydrogen carbonates in the pond water take on a buffering function and thus prevent abrupt changes in the pH value. If the proportion of hydrogen carbonate ions is too low, life-threatening reduction (acidity drop) of the pH value can occur for many fish and plants. The hydrogen carbonate ions are in balance with the carbon dioxide dissolved in the pond water. The carbon dioxide is of essential significance as a plant nutrient source for photosynthesis. Consequently the correct carbonate hardness is also extremely important for plant growth.

The ammonium ion is the result of nitrogen mineralisation. The organically bound nitrogen (e.g. in proteins) in food remnants and excrement is transformed by protein-splitting bacteria and released in the form of ammonium ions. Depending on the pH value, a balance between ammonium ions NH4+ and ammonia NH3 is present in the water. At pH values less than 7, ammonium ions predominate, at pH values greater than 7, ammonia increasingly occurs. Ammonia is extremely dangerous as it impairs the respiration of pond fish and blocks vital functions. Water plants use ammonium as a source of nitrogen. In a healthy and run-in pond filter ammonium is quickly oxidised to nitrate via nitrite by nitrifying bacteria. If this process chain is impaired a sudden increase of ammonia / ammonium can occur.

The ammonium ion is the result of nitrogen mineralisation. The organically bound nitrogen (e.g. in proteins) in food remnants and excrement is transformed by protein-splitting bacteria and released in the form of ammonium ions. Depending on the pH value, a balance between ammonium ions NH4+ and ammonia NH3 is present in the water. At pH values less than 7, ammonium ions predominate, at pH values greater than 7, ammonia increasingly occurs. Ammonia is extremely dangerous as it impairs the respiration of pond fish and blocks vital functions. Water plants use ammonium as a source of nitrogen. In a healthy and run-in pond filter ammonium is quickly oxidised to nitrate via nitrite by nitrifying bacteria. If this process chain is impaired a sudden increase of ammonia / ammonium can occur.

In the presence of oxygen, nitrite ions (NO2-) and nitrate ions (N03- occur in a decomposition process from nitrogen compounds, referred to as "nitrification”. In high concentrations nitrite is a toxic intermediate stage for fish and molluscs. Its physiological effect causes an inhibition of oxygen transport in the blood and prevents cellular respiration. As a rule, the nitrite that occurs from ammonium is quickly oxidised to comparatively non-toxic nitrate through nitrifying bacteria. If this bacterial oxidation process is inhibited, e.g. by new filter material, an unfavourable increase in the nitrite concentration can occur.  Nitrite is the end stage of the nitrification process and influences the life and well-being of fish and molluscs to a slight extent. Sources of nitrogen compounds in the garden pond are primarily fish food remnants, excrement and organic decomposition products. Usually nitrate concentrations that occur in the pond are not toxic, however at increased concentrations increased algae growth occurs.

In the presence of oxygen, nitrite ions (NO2-) and nitrate ions (N03- occur in a decomposition process from nitrogen compounds, referred to as "nitrification”. In high concentrations nitrite is a toxic intermediate stage for fish and molluscs. Its physiological effect causes an inhibition of oxygen transport in the blood and prevents cellular respiration. As a rule, the nitrite that occurs from ammonium is quickly oxidised to comparatively non-toxic nitrate through nitrifying bacteria. If this bacterial oxidation process is inhibited, e.g. by new filter material, an unfavourable increase in the nitrite concentration can occur.  Nitrite is the end stage of the nitrification process and influences the life and well-being of fish and molluscs to a slight extent. Sources of nitrogen compounds in the garden pond are primarily fish food remnants, excrement and organic decomposition products. Usually nitrate concentrations that occur in the pond are not toxic, however at increased concentrations increased algae growth occurs.

Phosphates are important nutrients for all plants and are indispensable for the metabolism of every living cell. Phosphates get into pond water through fish food, decaying vegetation, and decomposition products of organic matter. But they are also added by water utility companies or they get into the pond from surrounding agricultural areas. A dynamic balance between supply and demand will prevail as long as the phosphate quantity is reduced to a low level by the available plants. It is only the uncontrolled increase in the phosphate content that causes problems, such as strong algae growth. Phosphates are considered a limiting factor for the growth of string algae.

Some water supply utility companies add chlorine to tap water for disinfection. Even in extremely low quantities chlorine is intolerable for pond fish and other pond inhabitants and must be removed from the water before the water is filled into the pond.