Wonderful, wonderful water! By Andy McConnachie from Aqua Landscape Design

Start to understand water and you start to understand what is happening in the intricate world of your garden pond

For many the word chemistry brings back unhappy memories of Bunsen burners, challenging equations and the periodic table. But understanding the basics of water chemistry and the nitrogen cycle is beneficial to any pond or aquarium owner.

Water is a truly wonderful substance. Each molecule is made up of two atoms of hydrogen and one of oxygen. Physically it should be a gas but because the oxygen atom is slightly charged it attracts and sticks to other water molecules and forms a liquid. This property also enables other molecules to dissolve in water, affecting the suitability of the water for aquatic life; pH and KH are two properties which can be affected by these dissolved molecules.

pH is a measure of acidity, the lower the value the more acidic the water, and the higher the value the more alkali it is. Pure water has a pH of 7 which is known as neutral; whereas tap water tends to have a pH of between 7.5 and 8 making it slightly alkaline. Most pond fish and amphibians prefer the water this way but some plant species prefer more acidic conditions. Rapid changes in pH can cause stress and even kill aquatic animals, so it is often useful to monitor KH which measures the temporary hardness of water, an important factor which helps to stabilise pH. KH is a measure of the carbonate levels in the water; generally water from around this area has a KH value between 8°d and 12°d which means it is good at buffering and therefore stabilising the pH. ‘Total hardness’ or GH is what makes the water ‘hard’ or ‘Soft’ and is related to the presence of calcium and magnesium salts in the water. Water hardness is known to
have an effect on the colouration of Koi, soft water brings out the red or ‘Hi’ colours, whereas in hard water the black or ‘Sumi’ colours will develop quicker.

The properties of water also change with temperature. The oxygen levels decrease as water temperature increases, which can cause problems for aquatic life during a long, hot summer. Low oxygen levels also mean increased relative CO2 levels which will then cause the pH to drop. In general, larger water volumes are more stable than smaller ones as they are able to buffer the effect of temperature and chemical change.

The Nitrogen Cycle

The nitrogen cycle is an important natural process for aquarium and pond owners to grasp as it explains why you can only keep a certain amount of fish in a fixed volume, and why activities such as water changes and filter cleaning are sometimes needed to keep fish healthy. It starts with the food that you feed the fish. Once digested the fish then secrete ammonium as a waste product.

Ammonium is also a product of decomposition; rotting plants, dead fish and uneaten fish food will also produce this. Ammonium itself is not very toxic but depending on other water conditions it often converts into ammonia which then causes damage to the skin and gills of the fish.

Naturally occurring Nitrosomonas bacteria live in the water and convert ammonia into nitrite which is less toxic than ammonia, but at high levels will prevent haemoglobin from being able to bind with oxygen. This means that the fish will find it harder to ‘breathe’. Nitrite is then converted into even less harmful nitrate by Nitrobacter bacteria.

Pond plants will feed on this nitrate, therefore cleaning the water for the pond inhabitants. Installing a filter into your pond is a good way to provide a home for all the helpful bacteria which run the nitrogen cycle; the water movement produced by the pump will also increase oxygen levels and you will be left with happy fish in a healthy pond.

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