Biological filters rely on Nitrifying bacteria to strip the toxins that Koi produce out of the water and then oxidize them into virtually harmless byproducts. Since all bacteria are a living organism, it needs two basic things, food and shelter. Their shelter is your biological filter and their food is the Ammonia that Koi excrete.
Nitrifying bacteria need time to multiply and colonize your filter. This is known as cycling. It usually takes 4 - 6 weeks for Nitrifying bacteria to build up adequate numbers in order to keep up with the fish load in the pond. There are two types of Nitrifying bacteria, Nitrosomonas, which takes care of the Ammonia and Nitrobacter, which takes care of Nitrite.
Before you put your Koi into your pond, your biological filter is dead. There are no nitrifying bacteria living on the media. That means if you put Koi in your pond, the toxins are not being oxidized and are building up. If no action is taken, the toxins will harm your Koi.
Now while the bacteria is growing and colonizing your filter, the Ammonia level will continue to rise until there is an adequate amount of Nitrosomonas. When there gets to be enough of them in the filter, the Ammonia level will reach its peak, or spike, and it will start to fall. It will be slow at first, but will pick up momentum as more and more bacteria grow. Soon you will end up with more than enough Nitrosomonas bacteria that's needed to eat up the ammonia, so the number of them in your filter will drop a little after the spike, unless you keep on adding more ammonia. The same thing will happen for Nitrite with the Nitrobacter bacteria.
If you didn't know, Nitrobacter turns Nitrite into Nitrate. Nitrate isn't toxic in low levels, but elevated levels can be immunosuppressant. Nitrate is controlled with water changes and/or large amounts of vegetation.
If you don't pre-cycle a biological filter, your Koi will have to go through the Ammonia and Nitrite spikes.
There are two things that you can do to prevent this from happening:
Buy some cheap feeder goldfish and put them into your pond to pre-cycle the filter.
Put Ammonia in your pond to pre-cycle the filter.
The best option is to put ammonia in the pond. Cheap feeder goldfish can be loaded with parasites and it's not to nice to make them site through the Ammonia/Nitrite spikes.
How to pre-cycle a filter
Add plain Ammonia to the water, enough to bring the level to 5 - 8 ppm. Add more or less Ammonia depending on the initial fish load. Remember, it is better to grow more Nitrifying bacteria than you need. Make sure that there are no additives in the Ammonia that you use that could harm your Koi. Also, make sure that there are no fish already in the pond. Ammonia is poison to fish.
Be wary of any pond products that claim that they can accelerate the cycling process.
The cycle is nature’s way of cleaning up organic toxins. Nitrifying bacteria are a slow growing bacteria and nothing in a jar or a box will make them grow faster.
Make sure that you have a test kit handy and check the water quality often.
You will need to be able to test for:
After a while you will notice that the Nitrite levels will rise and the ammonia levels will fall. That means that Nitrosomonas is growing in your filters. The ammonia level will continue to fall until it is gone and you will be left with a large amount of Nitrite in your pond. After a while you will notice that there will be a buildup of Nitrate in the water. This is Nitrobacter doing its job, its turning Nitrite into Nitrate. When all the Nitrite is gone, the cycle is over and your biological filter is ready for its intended fish load.
Before you add any fish, do a big water change to get rid of the Nitrate. If you aren't able to put Koi in your pond right away, add more Ammonia to keep the Nitrifying bacteria alive.
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