Nitrates In Pond Water Cause Blanket Weed Problems

Quite often, nitrates are overlooked completely. Once the nitrification process is taken care of by our friendly somewhat temperamental bacteria, the end result is no ammonia, no nitrites and a whole bunch of nitrates instead. Whilst nitrates are largely harmless new thinking leads to the belief that in excess quantities they can be linked to a number of fish diseases.

Typically nitrates are used by plants as food. Algae is a plant and hence excess algae growth in a pond can, but not necessarily, be linked to a high nitrate concentration.
Good algae growth, the short manageable stuff that grows on the sides of the pond walls is in fact to be encouraged. It gives the Koi something to browse on, it removes some of the nitrates in the pond and it contributes to the stability of water quality in the pond. Too much algae on the other hand is a problem and the dreaded blanket weed can make pond maintenance a royal pain and quickly rob the Koi keeper of much pleasure.

Nitrates can also be broken down by bacteria into nitrogen gas. This is a little more easily said than done however. This is called the denitrification process.

The key to this process is that it takes place under anaerobic conditions, something which Ive stressed is not something that we want in our Koi ponds or in our filter systems.

The process is once again a two step affair. The first step involves undoing all the hard work of the Nitrobacter by converting nitrate back into nitrite. The second step involves converting the nitrite into 3 different gases namely nitric oxide, nitrous oxide and nitrogen gas.

The problem with dealing with nitrates in Koi pond is that it simply isnt practical to build a denitrifying filter large enough to do the job as is done in aquariums. You also need a constant organic carbon source, which would mean keeping some of the solid sludge material in the filter. It is difficult in the extreme to keep control over.
Nitrate build up in a pond is thus most commonly handled by water changes, and the build up of beneficial algae, or by using Kaldnes bio media. A popular trend is to make use of a so called vegetable filter which is a natural way of using up nitrates by making use of plants for which nitrates are an ideal food source.

You have to take some care with vegetable filters. Koi, remember will eat just about anything and a tasty plant (to a Koi ALL plants are tasty!) will go down very well thank you. Ideally you want a plant that can make do with as little soil as possible you dont want soil getting into your pond and placing unnecessary load on your mechanical filtration. You also dont want a large anaerobic volume which is what a big bed of plants and soil will become. Soil can also become a breeding ground for all sorts of parasites and you certainly dont want this in your pond.

But as with all things in Koi keeping there is a balance. If you design your pond correctly a number of planting options can become available all of which are imminently suitable for Koi keeping without the risks. Existing ponds are also not beyond help.

The danger of nitrates beware!

If your pond suffers from low oxygen levels, the denitrification process can take place quite naturally, especially in a gunge filled filter with the result that nitrite is fed straight back into the Koi pond. This is most common when conditions are warm (remember that warm water dissolves less oxygen) and at night when algae take up oxygen instead of releasing it.

Heterotrophic bacteria accumulating with any rotting solid organic materials will switch to nitrate reduction overnight, releasing all these poisonous nitrites into the pond only to reverse the process once more when oxygen levels return to higher levels with the onset of photosynthesis i.e. daylight.

The message is clear: Be rid of solid wastes to be rid of any potential problems. Also, oxygen levels are once more demonstrated as being of critical importance in a Koi pond, and especially so when its warm.

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Article by William Kelly