Learning the Nitrogen Cycle...

The Nitrogen Cycle in summary with pictures.

It is important to understand this concept clearly - so here is how the nitrogen cycle works

It is just as well that Nature has provided aquatic life with the Nitrogen cycle. Also called the cycle of life, the nitrogen cycle is a vital part of successful Koi keeping.

A picture says a thousand words. Below is a simple schematic of what transpires in the nitrogen cycle.

The Nitrogen Cycle

As we can see the Nitrogen cycle is a true cycle in that one process feeds another.

Koi, or any fish for that matter, produce ammonia as a result of their metabolic processes. Ammonia is highly toxic in even small quantities and only a little is needed to produce concentrations that are lethal to fish.

Fortunately, bacteria known as Nitrosomonas utilize ammonia as a food source breaking it down into nitrites. Nitrites are almost as bad as ammonia as far as fish toxicity is concerned and it's just as well that nitrites form a food source for a second bacterial grouping called Nitrobacter. Nitrobacter break nitrites down into nitrates, which are relatively harmless when compared to ammonia and nitrites.

And that's where the nitrogen cycle from a bacterial point of view ends. Nitrates are however a source of plant food and this is the primary means of removal in a natural pond or lake. In a Koi pond however, nitrate build-up can present serious problems. Water changes are an expensive way of preventing nitrate build up but can often be the only solution in some situations.

Some Koi ponds incorporate vegetable filters that are out of the way of Koi since Koi tend to eat plants as quickly as they are put in the pond. Also, the natural algae build up that occurs helps to keep nitrates under control. That said, levels higher than 100 mg/l are considered dangerous and should be reduced as quickly as possible to safe levels of around 50 mg/l.

What perhaps isn't clear from the diagram is the fact that the reduction of the ammonia to nitrates requires oxygen. In fact for each ammonia unit that is broken down, 4.3 units of oxygen are used. Without oxygen the nitrogen cycle cannot take place successfully.

Nitrates in high concentrations can become a food source for anaerobic (low oxygen conditions) bacteria. Generally speaking this is not a desirable situation since these anaerobic bacteria tend to produce dangerous toxins in the process. In fact, nitrates can be used in the production of nitrites under low oxygen circumstances - one of the very things we're trying to be rid of!

The bacteria used in the nitrogen cycle are also relatively slow growing when compared with other types of bacteria. A bacterial colony of sufficient size to cope with the ammonia load in a Koi pond full of hungry Koi can take many weeks to establish and during this time it is entirely possible that Koi many be exposed to dangerous ammonia levels, sometimes even fatally so. Cleaning out your bio filter carries this risk and you need to be aware of it.

The Nitrogen cycle and the bacteria (known as nitrifying bacteria) are dealt with extensively under this section of the website (biological filtration processes). Carry on browsing through the site and learn some more...