Protein skimmers are a nice to have optional piece of equipment. They shouldn't be. On any serious Koi pond a protein skimmer should be installed as a matter of course.
This is because protein skimmers remove a tremendous amount of waste over time. A Koi pond stocked with Koi will produce dissolved proteins which will accumulate over time. Sometimes you see this as a froth on the surface of the Koi pond and such froth is evidence of a high level of dissolved proteins in the Koi pond.
The way a protein skimmer works is quite simple. All that is needed is a column of water that is highly aerated with as small a bubble as possible. Dirty water is passed through the column and as it does so the bubble in the system tend to bind dissolved proteins to the surface of the bubble.
As the proteins build up a foam is formed which then comprises of protein that is now no longer dissolved. The foam should be quite stiff and as it builds up it will overflow from the water column, where it can then be easily removed to waste.
You do not have to have a high turnover rate of water through a protein skimmer - once every eight hours or so is normally sufficient. Protein skimmers also work at different rates at different times of the year. Often a protein skimmer when first installed works continuously for a few days spewing forth tons of rubbish. After the initial cleansing it would appear that the unit is not working and many people turn them off. You can do this and only run your protein skimmer once a week for a day or so, but our preference is to run them 24/7 and especially so in summer when your Koi are feeding heavily.
Removing proteins from the water body of the Koi pond means healthier water because all the proteins are removed and not just the proteins that would otherwise just be partially changed out with your regular water changes.
Protein skimmers add a measure of clarity to your Koi pond - water that is properly skimmed has a tendency to get that sparkle and luster to it. In addition skimmers can also remove very fine particulate matter from the water body - but this does depend to a large extent as to what this particulate matter is made up of.
Note that protein skimmers are not pressurised filters and can only be of the pump fed and gravity return type filtration system design.
Some pictures of a protein skimmer in daily operation:
Here you can see the recirculating pump with the venturi attached. It sucks water from the column and returns the water back to the column infused with billions of bubbles. As such it is a closed circuit.
The inlet of dirty water on the left, with the outlet return water being the tee piece on the right.
On the right had side you can see the waste discharge pipe - which in this case spews into a bucket which is then emptied when needed. You can clearly see the discharge foam built up in the bucket.