Inspired by the contribution to the filter basics(birdman 03-07-2009) I would like to say something about the pond basics.

Hourly circulation
A koi pond should be circulated once an hour. The dirt will quickly get into the mechanical filter and the biological filter will work more effectively.
In a 20,000 litre (5000 gallons) pond, 20,000 litres (5000 gallons) should also be pumped effectively. Pay attention to the characteristics of the pumps, because a 5000 gallon pump will only pump 5000 gallons under optimum conditions. As soon as a hose or a pipeline is connected, a counterpressure is created and the pumping capacity drops.

Bottomdrain (BD)
A BD draws only within a very small radius. The sediment enters the BD via the current and not through suction. At least this is rarely the case.
We in Europe actually have only 4" BD's or larger where 4" is the standard. About 10,000 liters (about 2500 gallons) have to be moved continuously to make it difficult for dirt to settle in the pipeline. From time to time a floor drain should be flushed, i.e. further drains or skimmers should be closed and the water is only drawn over a drain.

Skimmer
We use here almost exclusively tube skimmers and only rarely skimmers which are built into the wall. The pipes from the skimmer to the filter, as with the BDs, are usually made in 4". Here also applies that about 10,000 liters (2500 gallons) should be drawn.

Hydraulics
With the average pipe length existing between the filter and the pond and a circulation of 10,000 litres (2500 gallons) per 4" pipe, the difference in level from the pond to the filter is approx. 3-4cm.
I therefore install 2 BD and a skimmer at 20,000 litres (5000 gallons). The difference in level decreases and the skimmer pulls a little less, but still sufficient to keep the surface clean. In this case, pumps are installed that pump 25,000 litres (6500 gallons).

mechanical filtration
The mechanical filter separates visible dirt particles from the water. The finer the dirt filtered from the water, the better the biological filter can work. Why this is so, I explain in the part about biological filters.
The most important nutrients for algae are phosphorus and nitrogen. These two elements occur in many chemical compounds in the pond and promote the growth of thread algae and algae bloom. Fish food contains a relatively high amount of phosphorus, a very good nutrient for algae, but it must also be present in the food. It should therefore be kept as low as possible. It should also be noted that about half of the phosphorus is excreted by the fish again, is then bound in the faeces but is released within 24 hours from the faeces and is then available to the algae. A quick dirt removal is therefore very effective to keep the phosphate content in the pond low.
Nitrogen is quickly excreted by the fish in the form of ammonium (about 65%). The rest is excreted via the faeces as undigested protein. An effective dirt separation should therefore not be missing in any pond.

biologische Filtration
In dem biologischem Filter sollte das anfallenden Ammonium über Nitrit zu Nitrat umgewandelt werden. Die bekanntesten Arten dafür sind die Nitrosomonas und Nitrobacter. Es gibt aber noch viele weitere Stickstoffverwerter. Die autotrophen Bakterien ernähren sich von gelösten Stickstoffverbindungen. Autotrophe Bakterien benötigen relativ viel Zeit um sich zu vermehren, Heterotrophe Bakterien hingegen vermehren sich deutlich schneller und machen so den wichtigen Autotrophen Bakterien das Leben schwer. Heterotrophe Bakterien ernähren sich von Organischem Material und das ist auch der Grund warum ein effektiver mechanischer Filter dem biologischen Filter die arbeit erleichtert.
Bei 10Grad Celsius benötigen Autotrophe Bakterien für eine Verdopplung der Kolonie etwa 38 Stunden, Heterotrophe Bakterien nur etwa 20-30 Minuten.
Die Stickstoffausscheidungen der Koi lassen sich übrigens auch sehr gut berrechnen.
Als Beispiel nehme ich wieder unseren 20.000 Liter (5000 Gallonen) Teich, in dem täglich 1kg Futter mit 40% Protein gefüttert wird. Es entstehen so täglich 2,7mg/L (2,7 PPM) Ammonium (NH4). Aus den 2,7 PPM werden 6,8 PPM Nitrit (NO2) und anschließen 9,2 PPM Nitrat (NO3).
By the way: just because a filter medium has a high surface area does not make it a good filter medium. After all, what good is the high surface area if the water cannot reach all places well?

chemical/physical filtration
There are different possibilities to treat the pond water chemically/physically.
For example, activated carbon can reduce the ammonium value, but nothing (very little) can change the nitrite value. But ozone can oxidize nitrite to nitrate but not change the ammonium value.
UVC on the other hand is not suitable to remove ammonium or nitrite, but very suitable to keep the algal bloom in check, to reduce the germ density and also to destroy residual ozone.
UVC radiation (exactly 253.7nm) penetrates a maximum of 20cm (8") into clear water, but the effectiveness is only usable for our koi ponds about 10cm (4"). The radiation intensity decreases enormously afterwards.
UVC should penetrate as clear water as possible, therefore it is recommended to use it behind the mechanical filter. In pumped ponds, however, this is usually not so easy. By the way, UVC rays are not reflected by stainless steel or glass, the bluish light reflects back but the wavelength is changed by the reflection.

Water values (nitrogen compounds)
Ammonium (NH4) should not be detectable. 0.05-0.1 PPM is good. Permanently, the value should not exceed 0.1PPM.
If ammonium is permanently too high, the following points should be considered: 1. reduction of the coi stock, 2. increase water exchange rate between pond and filter (but the change can take several weeks), 3. reduce feeding.
Nitrite (NO2) should not be detectable like ammonium and should not exceed 0.05-0.1 PPM. Nitrite is always formed in the course of nitrification and is therefore directly related to ammonium. If the nitrite content is permanently too high, the same measures can be applied as for ammonium. Salt (iodine-free) helps to minimize the toxicity of nitrite. 100g per 1000Liter are sufficient but this is not a permanent help.
Nitrate (NO3) cannot be further degraded. Lowering the nitrate content requires a partial water change. However, the initial water should also be measured here, because often the nitrate content in the water is already quite high. Nitrate is relatively harmless. Concentrations of 20-100 PPM are completely in order.

temperature
The optimum temperature curve varies according to the region.
In general, the temperature fluctuations should be as low as possible. My pond is heated with an air heat pump and cooled if necessary.
In winter (January-March) I keep the temperature at 8 degrees Celsius, increase it to 16 degrees Celsius in March within one week and keep it until the beginning of May. Then I increase within 2 weeks to 25 degrees Celsius and hold the 25 degrees until mid September. The drop in temperature to 16 degrees lasts for two weeks and I keep the 16 degrees until the beginning of December and then go back to 8 degrees within 2 weeks.