Check out Your Mechanical Filtration. Does it measure up?
Mechanical filtration takes place normally at the very start of your Koi pond filtration system. It's role is to remove any solid (organic) material from the system before the pond water reaches the sensitive biological filtration stage.
Organic wastes that reach the bio filter stage are not beneficial in any way. An ideal mechanical filter removes ALL solid wastes so that the biofilter can operate with almost exclusively nitrification bacteria and hence optimise the efficiency of the ammonia breakdown process known as the nitrogen cycle.
There are a few ways of achieving this process. In human swimming pools, sand filters are highly effective at trapping solids for easy backwashing and rinsing away and there seems to be a disturbing trend, particularly in South Africa of using these on Koi pond systems.
However, there are some major differences between swimming pools and Koi ponds, the biggest being that Koi ponds are supposed to keep things alive. Sand filters have no place in Koi ponds and especially not as a primary mechanical filtration devices. Find out why here!
The other primary means of removing solids is by using a vortex chamber. These are typically big, the bigger the better and pond water is allowed to slowly circulate in the chamber, allowing enough residence time for solid particles that are denser than water to sink and collect at the bottom from where they can be purged.
Vortexes make the dangerous assumption that all solids are denser than water. This is often not the case - leaves, gunge, rotting dead algae killed by the UV etc often floats. In many more instances the solid materials density is so close to that of water that even in a massive vortex the particles simply do not have enough time to settle out.
Heterotrophic bacteria that can negatively compete with the nitrification bacteria in the biofilter (see inorganic wastes) don't mind what form their organic solid food takes. As long as it's there, they'll pitch up for the meal. And pitch up in great numbers at that, competing with and hampering the performance of the much needed nitrification bacteria.
When heterotrophic bacteria appear in great numbers so do pathogens and unwanted, harmful bacteria (to Koi anyway). You may not see them, but your Koi know they're there and at the first sign of weakness an apparently otherwise healthy Koi can easily be overwhelmed and succumb to a secondary infection that would never have troubled it ordinarily. Something simple like a cut or a graze, handling wound or a even bird peck becomes potentially life threatening. Minor cuts and damage should be no problem for a healthy Koi in a healthy pond!
Often two or three vortexes are linked in series to enhance their efficiency. However, the volume of waste material gathered in the second or even third chambers is often not sufficient to justify the additional cost save to all but the hardened Koi purist. Normally a bigger first vortex would perform better overall.
There are many variations on mechanical filtration involving brushes, foam blocks, and so forth and so on. A quick browse on the Internet will provide you with many such solutions - Japanese matting incidentally is NOT used for mechanical filtration - it is a bio media and acts as a biological filter. It is not operating at optimum efficiency if it is not free from organic debris.
There are two problems with this style of mechanical filter. The first is that of maintenance. As they collect solid debris much like they're supposed to, they have to be regularly cleaned to maintain their efficiency and to get rid of the muck that they accumulate. This should be done ideally every day if you want to avoid high bacterial counts - heterotrophic bacteria colonise alarmingly quickly.
Secondly, no matter how fine the medium, a percentage of solid material is going to make it straight through the media. These are typically the smaller particles that have been broken up or break off from the media as bacteria munch away on them. A significant proportion thus finds its way into the biological filter, which, being cleaned far less often (because the nitrification bacteria take a long time to colonise you do not want to disturb them more than is absolutely necessary - ideally never) than the mechanical filter material.
Once in the bio filter there is a two fold effect. The heterotrophic bacteria quickly build up in numbers and start competing for space with the nitrification bacteria, as well as adding to the ammonia load. Now, because this is taking place you are also forced to clean out the bio filter media more often causing severe disruption to the nitrification bacteria colonies and hence to the overall nitrification process. This is turn can be severely problematic for your Koi and about the only solution is to reduce or halt feeding, and then gradually building up again as the beneficial (nitrifying) bacterial colonies take hold once more.
And then you try telling a pond full of hungry Koi that the diet is for their own good...
Organics in the bio filter are thus a double whammy and one you and your Koi can live without. Over the years the benefits of brilliant mechanical filtration as offered by the Answer easily outweigh the initial cost. It simply has no equal.