Biological Filtration in Koi Ponds

Biological filtration is not a difficult concept to understand. It's at the heart of the nitrogen cycle and is responsible for removing all the dissolved wastes that would otherwise accumulate in your Koi pond and kill all your Koi. If left unattended, dissolved wastes in the form of ammonia can wipe out a Koi population in a matter of days.

The nitrogen cycle is explained under Inorganic wastes  - if you are not familiar with it read this section now before proceeding.

An effective nitrogen cycle, one in which all ammonia is completely broken down into much less harmful nitrates requires an effective 'reaction'. It's elementary chemistry and the only uncertainty in the entire cycle is the nitrifying bacteria themselves. If they're not present, the 'reaction' cannot take place. If there are not enough of them present, then the reaction cannot proceed to completion - in other words, not all the ammonia and nitrites will be 'reacted' into nitrates, and the pond water will have to make a second or third pass through the filter.

I have stressed the importance of fish stocking densities as much as I can. It cannot be over emphasized enough.

Koi are tough fish. But with a vastly increased density per cubic meter of water as we find in Koi ponds, the stresses placed in the quality of the water in which they live are enormous. After feeding, ammonia levels in the pond will greatly exceed those as found in a natural lake or Koi mud pond. This is a situation that Koi can tolerate but should not have to! Prolonged exposure to ammonia is dangerous and will wear the Koi down until eventually an opportunistic bacterial infection or disease breaks out and kills the Koi (which under normal high water quality circumstances it would have been to easily resist).

The point should be clear that the role of the biofilter is to remove as much of the ammonia as it can as fast as possible. Speed is everything in this process and it is vital that the bio filter be able to remove ALL the ammonia and ALL the nitrites produced in a single pass of pond water. It simply is not good enough if any nitrite or ammonia makes it back into the pond.

Why is this so important?

If you consider a Koi pond of say, 10 000l. The filtration system should turn over this pond volume once every two hours as a rule of thumb. This means that if all the water in the pond passes through the filter system once, it will take 2 hours to reduce the ammonia concentration down to zero.

Note that it is a dangerous assumption to make that the water is perfectly evenly distributed and that each liter will make it through the filtration system every complete cycle!

It gets even worse though. If the ammonia concentration at the start of the filtration process is 100 mg/l by way of example let's see what happens over a two hour period in our 10 000l pond. At the start of the process, in a Koi pond of 10 000l we will have a total of 10 000 x 100 = 100 000 mg of ammonia.

After 24 minutes, 20% of the pond volume (2 000l) will have been circulated through the filter system. Assuming we have a bio filter that is capable of completely removing all ammonia and nitrites converting them into nitrates (plant food) the amount of ammonia in the pond will have dropped by 20%. It will now be
2 000l x 100mg = 20 000mg less than it was before.

This means that the concentration of the ammonia in the pond will be reduced to 80 mg/l. Now consider what happens over the next 24 minutes.

Again, another 2 000l of pond water passes through the filter system, and all the ammonia in this water is removed. Because the CONCENTRATION however is now LOWER (at 80 mg/l) the MASS of ammonia removed is LESS. 2 000l x 80 mg/l = 16 000mg of ammonia will have been taken out over this time interval.

Now the concentration of ammonia in the Koi pond is our starting mass of 100 000mg less the mass removed in the first 24 minutes of 20 000mg and less the mass removed in the next 24 minute interval of 16 000mg, which leaves us with a total of 64 000mg still in the pond water.

This is a concentration of 64mg/l.

Can you see that the rate at which ammonia is removed depends on the concentration of the ammonia in the water to begin with? It will take considerably longer than 2 hours, even in a perfectly mixed pond to reduce the concentration of ammonia to zero.

If even only a little ammonia is recycled to the pond from the bio filter because there is inadequate bio filtration taking place, the time taken to reduce the ammonia to zero will increase dramatically. This increases the exposure period of the Koi to the ammonia, as well as the concentration levels that the poor Koi have to suffer with.

There is one obvious solution which is to increase the rate of turnover through the filtration system. However, if the water passes through the filter system too quickly, the bacteria will not have enough time to munch all the ammonia - it will simply pass through too quickly.

Yes, you can run two or three systems in parallel and increase your performance that way. However, consider the practicalities and the expense of doing this. Bigger pumps mean more electricity. More biofilters mean more expense. More maintenance. Etc. No doubt there are sufficient fanatics out there who will take the hobby to this level and I encourage them to do so, but for the average Koi keeper this is simply not a practical route to adopt.

As I've also said before, Koi are tough. The only time you'll know anything is wrong will be when it's too late. It may even take a few years - when the Koi are nice and big, and have had a good feed in the middle of summer when it's warm and they're eating like the pigs that they are and the bio filter hasn't been properly cleaned for a week or two and so forth and so on until a combination of circumstances fall into place that leads to disaster.

It only has to happen once.

Water quality is critical to Koi keeping. Biological filtration is critical to water quality. Superior performing bio filters that can expand to accommodate growing Koi and increased feeding levels are what I would consider an 'essential first purchase above all else' necessity.