Oxygen (part 1)

Oxygen (O) is a radically aggressive substance that doesn't actually occur as a single atom in nature. An oxygen atom (O) likes to bond with other atoms, often resulting in the destruction (combustion) of that other atom. When we talk about oxygen, we actually mean the oxygen gas molecule, which consists of two oxygen atoms (O2). However, we colloquially call that O2 simply oxygen, so we'll stick with that name.

The benefits of oxygen

Oxygen is, of course, an essential substance in a pond. Not only do our fish need it to metabolize food, but our biological filter also uses it heavily. A carp needs at least five milligrams of oxygen per liter to function optimally (O2: 5 mg/l). At this level, it can digest food and grow optimally. To get the most out of your biological filter, it's advisable to maintain a minimum oxygen level of seven milligrams per liter (O2: 7 mg/l). Nitrifying bacteria, which convert ammonia via nitrite into nitrate, work best at 7 mg/l. The dirtier the pond, the more oxygen is consumed by organisms you don't want. Decaying bacteria, for example, also consume a lot of oxygen, so keeping the organic matter pollution in the pond low makes it easier to meet the oxygen standard.

How does oxygen get into the pond?

The pond absorbs oxygen through diffusion. Diffusion describes how molecules are constantly in motion, moving from a place of high concentration to a place of low concentration. Therefore, when oxygen is used in the pond, it is automatically replenished from the surrounding air. Oxygen exchange between air and water therefore occurs at the surface. However, diffusion takes time, so when a lot of oxygen is used in the pond, a shortage can occur because it is not replenished quickly enough by oxygen from the air. This can be a problem, especially with warm water. The solution is simple: increase your water surface area by creating movement (for water pumps , air pumps , wind, rain, a waterfall (and so on). "Rippled" water has a larger surface area than a flat surface. This allows for greater oxygen exchange through diffusion. Therefore, aerate sufficiently in the summer to retain at least that 7 mg/l.

Absolute and relative oxygen content

The oxygen content in a pond can be expressed in two ways: absolute oxygen content, which expresses the amount of oxygen in milligrams per liter, and relative oxygen content, which indicates the percentage of oxygen relative to the maximum possible oxygen concentration (given the temperature) of 100%. This relative oxygen content allows measurements to be compared (regardless of temperature). For example, a pond with a relative oxygen content above 80% can be considered hygienic, while a percentage below 60% can be considered highly polluted, regardless of whether measured in summer or winter.

Cold and warm water

It's a physical fact that cold water can hold more oxygen than warm water. This means that meeting the minimum oxygen saturation requirement of 7 mg/l is much easier with cold water. For example, the maximum saturation at 10 degrees Celsius is 11.3 mg/l, while at 24 degrees Celsius it's only 8.4 mg/l. The warmer the water, the less oxygen it can hold, meaning that above 20 degrees Celsius, an unsanitary pond poses a real risk of oxygen deficiency. If your relative oxygen content is 70%, for example, you can still meet the requirement at 10 degrees Celsius (70% of 11.3 mg/l is 7.9 mg/l), but at 25 degrees Celsius, you'll have a major problem (70% of 8.4 mg/l is 5.9 mg/l).