Filtration

Filtration is probably one on the most important things for keeping koi .
Below is a simple representation of a filter.
Filters Remove visible and invisible toxic waste from the pond.
Bacteria in the filter live off the wastes, the more bacteria living in the system, the more effective is the filter.
The pump must operate continuously.

The filter  below will filter a pond upto 2000 litres depending on position (sunlight or shade). - It has been cut away to show the internal workings.

It is based on a 200 litre plastic drum which you can obtain from a drum recycler for usually under $20..
The lid of the drum can be used for the false bottom - drill holes in the false bottom to allow the water to get through.
Use 90 or 100mm stormwater pipe for the centre pipe - make a few "V" notches or large holes  in the bottom of the pipe to allow the water to get into the cavity.
You need a hole just below the center pipe height  in the drum for the return water.
The inlet pipe is optional - a hose from the filter can be put in the top of the center pipe instead of drilling a pipe through the side of the drum
The filter medium can be anything that can grow bacteria - gravel, pebbles, onion bags, scouring pads etc

The final cost should be less than $50.

If you have a bigger pond you can use more than one drum or 1000 litre drum or make the filter out of the same material as the pond

9000 Litre Pond - Filter capacity 2000 Litres