Nitrates in Koi ponds

It is a common question. How does one remove nitrates from a Koi pond? After all we know our bio filters convert poisonous ammonia into poisonous nitrite and then into non poisonous nitrate.

Although nitrate is not toxic to Koi, over time constant exposure to high levels is not something that is advised. Mark sums it up best - nitrate in water is akin to the bad smell left over in a bathroom. After a binge of cheese, cabbage and beer


But what removes the nitrate?


Mostly nitrate is controlled by water changes. However, nitrate is often present in the source water that we use on our Koi ponds - which is one reason why it is important to check your source water every now and then. So what to do?

There are a number of remedies open to you. True, nitrate is a source of plant food and true, plants will absorb an amount of it. However, for plants to make any meaningful contribution to the nitrate battle you will need a lot of them. Roughly double the surface area of your Koi pond in fact. 

This makes plants in a Koi pond practically ineffective as nitrate removal devices.

Plants can in fact make your life somewhat more uncomfortable. Aside from your Koi wanting to eat them, plants, if not properly maintained weekly, will build up sludge and detritus in their root systems. This detritus and sludge will in fact ADD to your nitrate problem.

A de-nitrifying system can be built cheaply and easily. All that you need is a bed of crushed stone, some 300mm in depth. with a surface area of about 5% of the pond surface area. This is called a Deep Sand Bed or DSB. Flow water over the surface of this bed. Denitrifying bacteria will remove nitrate from the water flow - you do not need massive flow rates over this bed - your pond volume once every 8 hours or so will be sufficient.

Ensure you have a void below the stone bed and that you can drain the DSB completely every now and then to get rid of detritus build up.

It is important to ensure that the DSB is fed clean water - detritus fed in wth the source water is going to provide you with a maintenance nightmare.

It does mean an additional thing to maintain on your filter system.



You can also keep nitrates under control using a product called Viresco. This is a naturally occurring microbe that munches on nitrates. It is extremely efficient and when it removes the nitrates it effectively starves itself and dies out. It means occasional replenishment is needed. We have found that maintaining about 10% of the original dose every two weeks is sufficient to keep nitrates at close to zero levels. 1 gram per 2500 litres is the initial dose required after which 1 gram per 25 000 litres every two weeks is sufficient to maintain nitrate levels at close to zero. At that rate Viresco is cheap, quick and effective.