Nitrification cycle .. koi, gold fish ponds and pond filters

Making the pond safe for your fish. Poison to pond plant fertiliser using a biofilter ... In this article we discuss the Nitrification cycle in gold fish and koi ponds; what it is and how it works and under what circumstances it can fail. In the article about ammonia it was shown that this very poisonous chemical is produced by koi eating any food containing proteins. It was shown that at different levels of pH ammonia could be more or less poisonous but that it was always poisonous and that any fish keeper should strive hard to keep the level of ammonia down to zero levels in the pond. To achieve this means the pond system must have a fully functional, day in and day out, biofilter because this is the real home of the Nitrification cycle.

The Pond poison Is Ammonia and the fertiliser is Nitrate

Nitrate is the N on N:P:K fertiliser bags. Two specific types of bacteria accomplish the magical task of converting ammonia to nitrate in a fish pond environment. We must be truly grateful to these minute yet powerful bacteria.

With names like Nitrosomonas and Nitrobacter these bacteria have to be very special "beasts" and so they are. However a bit like Goliath they have a weak spot - they are useless without oxygen as far as we are concerned. For ease of typing we will now refer to Nitrosomonas and Nitrobacter as the two "Ns".

The two "Ns" are around us everywhere: they are in the soil and in the air. Gerry Preston whom you have heard me mention before even spits into the pond to add small amounts of the two "Ns". This pervasive presence of the two "Ns" means that almost as soon as a single fish is placed in a new pond the nitrification cycle gets started. But it ONLY gets started and it takes a long time to get into top gear as you will find out.

The Nitrification Cycle in some detail

For the sake of this explanation we are starting with a brand new pond which has just been completed, filled with water, and the biofilter and UV have been tested for leaks. The pump is working fine and consuming only a small amount of power because it is a FOCUS pump - couldnt resist that. An air blower happens to have been installed on this pond which was an excellent idea and air has been diverted to blow into the pond itself, into the biofilter and also into the chamber supplying the waterfall with a nice constant stream of water - in other words 3 separate air outlets.

Now we need a fish or two to be exposed to a potentially stressful couple of weeks while the ponds biofilter "matures." Initially the biofilter contains no bacteria and it will take between 4 to 6 weeks for the two "Ns" population to reach a level whereby more fish can be introduced to the pond without overloading the system and causing excessive amounts of ammonia to be formed. Most pond keepers buy low cost fish to see them through this stage. The reason we need a fish is to provide the starting food for the Nitrification cycle to kick in.

The moment this fish has been fed it secretes some ammonia (remember about 4% of the foods weight becomes ammonia). In the presence of oxygen which has become dissolved in the water the first of the two "Ns" is able to consume the ammonia and converts it into Nitrites. In this way the first of our two "Ns" namely N1 grows and multiplies - I believe it takes 18 hours to double in population - and provides more capacity for consuming ammonia. In the early stages however ammonia levels continue to build up faster than they can be removed because there just are not enough bacteria around yet to take up all the ammonia. If you tested the water during the first week or so you would see the ammonia concentration increasing daily and you would see very little Nitrite build up. You would certainly see no Nitrate build up although you may see some Nitrate which was already present in the tap water used to fill the pond. Always remember that your tap water almost certainly contains some Nitrates. Part 1 of the Nitrification cycle is now under way by courtesy of the first of the two "Ns" and Oxygen.

This is what is happening:

NH4 + O2 produces NO2 + 4H in very simple chemistry terms so please do not criticise me too heavily. This can be read as follows: Ammonium plus oxygen produces Nitrite and 4 bits of Hydrogen. It only does this in the presence of N1 - Nitrosomonas bacteria. The presence of the 4 Hydrogen ions will tend to lower the pH of the pond water. If insufficient Oxygen is provided then it is possible not all the ammonium can be consumed as quickly as it should be.

After about 10 days the ammonia level starts to fall and now we see the Nitrite level rising. After about 20 days the ammonia level has dropped considerably and after about 40 days or so all being well the ammonia is no longer detected. All the time this is happening the fish is producing more ammonia so the total nitrogen in the pond water is increasing steadily - some of this nitrogen is in the form of ammonia and some in the form of Nitrite and after about 12 days or so some of it is also Nitrate our fertiliser. At the stage that you see the Nitrate level increasing you are seeing stage 2 of the Nitrification cycle coming in to play. This is what is happening now:

2NO2 + O2 produces 2NO3. This can be read as follows: two portions of Nitrite come into contact with oxygen and the Nitrite is oxidised to Nitrate. This however can only take place in the presence of our bacteria N2 - Nitrobacter. As N2 consumes more Nitrite it grows big and strong or at least multiplies considerably and is soon able to eat all the Nitrite thrown at it. Around day 40 not only is there no ammonia there is also no Nitrite. The stage has been reached at which the level of bacteria and level of oxygen dissolved in the water is able to almost instantaneously convert all the ammonia produced by feeding the fish into Nitrate fertiliser. I think you must agree this is almost magic.

At this stage as far as we are concerned the cycle is complete and everything carries on marvellously until we introduce more fish. Suddenly the ammonia level rises again because we are producing more than the biofilter can cope with. The secret is to gradually increase the quantity of fish and give the two "Ns" time to get to grips with the extra amount of food coming their way.

What is important to note is that there may be no ammonia and no nitrite appearing in our tests but the total NITROGEN is increasing steadily. Fortunately the fish can tolerate as much as 3000 times more Nitrate than ammonia. However it is not a good idea to allow the Nitrate level to get too high and occasional changes of say 10% of the pond water are good for the pond environment. By this I mean pump out 10% and add a fresh 10% - evaporation is not the same as a water change.

Over the period of the first 12 months the pond will get better and better in terms of its ability to remove ammonia and a true steady state will be reached that will enable you to really enjoy wonderful water quality and wonderful fish growth. Just make sure you keep the pump running, the biofilter clean, aeration on and add fish in moderation. Also feed the very best food you can afford - you will not regret this if high water quality and great fish health and growth is your aim. Do not drink more than 6 beers while you sit next to your pond and enjoy what you have created - you might just fall in.

It is called the Nitrification cycle because Nitrogen is continuously recycled. Nitrogen comes into the pond via fish food, it goes out of the pond as a fertiliser and sooner or later finds its way back.

At 20 degrees Centigrade 0.9 gms of ammonia per sq. metre of biomedia per day is converted to nitrate. At 10 degrees it is only 0.5 - reference Aquatext Dictionary.

It should now make common sense that by having larger viable quantities of the two "Ns" in a biofilter the better the filter will work. It should also be blatantly obvious that pond aeration is not just a good idea it is very important for the serious koi keeper. Not quite so obvious but necessary is to get the oxygen and the bacteria and the food (ie ammonia and or Nitrite) all together at the same time in order to get the best results. This is the essence of good biofilter design which will be handled in a separate article.

To turn this beautiful theory into practice then a pond bio filter is needed ...

Do reconsider your intention to build a pond if you are not prepared to spend money on a pond bio filter. You will be wasting money if you go ahead without a bio filter and I promise you will do it sooner or later or you will turn the pond into a sand pit.

These principles are best incorporated into modern pressure filters made by Hozelock and Fishmate for small to medium sized fish ponds (NOT SAND FILTERS) and bead filters for large ponds. You can see range of pond bio filters to suit your pond here . Don't forget the turbulent action is created by the pond pump you choose

Alfagrog is called Supra in USA Alfagrog is called Supra in USA and can be bought here ... it is best all purpose media for small to medium ponds . Alfagrog looks a bit like cinders, it is lightweight, comes in different sizes and you can literally blow through it because it is so porous.