Chapter 15 / UNDERSTANDING LIVING WATER
pH, alkalinity and hardness answer different questions.
At a glance
pH tells you about the water's acid–base condition at the time of measurement. Alkalinity describes its ability to neutralise acid. Hardness mainly reflects dissolved calcium and magnesium. These ideas are related through water chemistry, but they are not three names for the same thing.
Plants and algae can move pH through the day by using dissolved carbon during photosynthesis. Respiration supplies carbon dioxide again. A morning-to-afternoon pH change may therefore be part of the pond's daily metabolism. The size and meaning of the change depend on the rest of the chemistry.
Begin with the source water and a reliable baseline. Repeated top-ups, evaporation, biological activity and added materials can gradually change the pond. Resist the temptation to correct every isolated reading immediately. A pattern linked to time, weather or maintenance is usually more informative than a single number without context.
The main idea: A buffering measurement helps explain pH behaviour; it does not lock pH at one value.
In practice
- Measure pH at a consistent time when building a trend. To investigate a daily swing, deliberately compare early morning and afternoon samples.
- Record alkalinity and hardness separately, including their units. Check the instrument or kit instructions rather than treating 'mineral content' as one parameter.
- Keep notes on source water, top-ups and materials added. If conductivity changes, use it as a clue to changing ionic content, not as an identification of a particular chemical.
Going deeper · the science behind this chapter
The carbonate system links dissolved CO₂, bicarbonate and carbonate through acid–base equilibria. Photosynthesis and respiration can alter dissolved inorganic carbon and pH without, by themselves, changing total alkalinity. Nitrification is different: its acid production consumes alkalinity. [S02, S03]
Alkalinity is commonly expressed as mg/L as CaCO₃. One milliequivalent per litre equals 50 mg/L as CaCO₃. Hardness may use the same reporting convention while representing a different chemical quantity. Sodium bicarbonate can raise alkalinity without adding calcium hardness; some calcium salts can increase hardness without a comparable bicarbonate supply.
For an illustrative alkalinity budget, oxidising 2 g ammonium-N consumes about 14.28 g CaCO₃-equivalent. In 20 m³ this is about 0.714 mg/L as CaCO₃, before considering other reactions and water exchange. Repeated loading can matter over time even when a single day's change is small. [S06]
Measure pH promptly because gas exchange can alter a stored sample. Follow a defined alkalinity titration method and record how filtered or unfiltered samples were handled when that distinction matters. Conductivity measures a broader ionic response and cannot replace the separate chemical tests. [S22]
Source water + carbon dioxide cycle + biological reactions + top-ups → the observed chemistry.
Related guides
Sources and further reading
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