What filtration to use?
Once you have decided where to locate your filtration and what pumps you are planning on using it is time to finalise what filtration you want to use on your Koi pond.
Typically in 99% of cases your filter system will be receiving water coming from the business end of a pump, whether this is a submersible or external pump. This is in contrast to some Koi ponds in which the filters are fed by bottom drains and pumps are located in the filters to return the water back to the pond.
As such the quickest and easiest option for filtration is to go for the pressurised filtration systems that are available on the market. These systems are generally easy to install, occupy the least amount of space, and require minimal maintenance. However they also have some significant drawbacks.
Let's take a closer look at pressurised systems.
Note that 'pressurised' is not really an accurate term. The system is in fact not under significant pressure at all - if everything is running as it should. But if a blockage occurs somewhere in the system preventing water from flowing freely then pressure as generated by your pump can build up in the system. In non pressurised systems by comparison this can't happen.
It might help you to think of a pressurised system as more of a closed system.
As with any filtration system the attack is two pronged - namely mechanical filtration followed by biological filtration.
With your swimming pool the chances are pretty good you will have a sand filter attached to the original filtration system. The temptation to use this on your Koi pond may be strong but we urge and encourage you to cease and desist. Sand filters have no place on a Koi pond, anywhere.
But you can convert your old sand filter into a bead filter. This is easily done and it transforms your sand filter from a dangerous obsolete dinosaur into a friendly, healthy and easy to maintain mechanical filter for your Koi pond.
A typical bead filter will be sufficient to handle more or less 20kl of pond volume. Because bead filters can take out really tiny particles of finer debris, in conjunction with a leaf trap that protects your pumps impellor from large debris your mechanical filtration requirements will be entirely covered.
It then becomes necessary to focus on your biological filtration. With a pressurised system these are very simple devices - essentially a tall tube filled with some media - typically bio balls - with an inlet and an outlet. Water flows into one end and out of the other end and as it does so it passes over the bio balls on which bio film grows.
From here the water is pumped through an ultra violet light and returned back to the pond.
Since the entire system is closed (or pressurised) the pump has to do all the work of moving the water through the system. Whilst the effects of friction through the various components may be minimal it does have an impact on the work being done by the pump. So your flow rates through these systems are always a bit lower than those on non pressurised systems.
The single biggest drawback of pressurised systems may not be immediately obvious but it is significant and one of the primary reasons why experienced Koi pond keepers tend to avoid using them. It is simply one of oxygen. At each stage of the filtration process oxygen is being sucked out of the water at a prodigious rate by all the bacteria living in the filters (these bacteria are truly oxygen sponges).
It means that the water being returned back to your Koi pond is oxygen depleted. Even returning water back over a waterfall or through a venturi is not going to significantly change this fact. It is a common misperception that that are oxygenation devices - sadly unless you have a waterfall a kilometre long you are not going to have water at saturated oxygen levels by the time it hits the pond.
If you have not yet read the page on aeration, do so here now.
Pressurised system are still widely used today. In most cases we prefer to use them when there are constraints of space, water returns and the like but our preference for Koi ponds is by far and away for low pressure/open systems, of hybrid systems that use a bit of both worlds.
Low pressure or open filtration systems are discussed here.