Koi pond filter

All the terminology gets a bit confusing, so let's start with some definitions. Typically, you'll have a koi pond filter system, in which you'll pass your pond water through several filter chambers containing different filter media, such as gravel, matting, etc. A good koi pond filter usually has more than one kind of filter media.

If you buy a professional koi pond filter, it may be a single plastic or fibreglass device with several chambers, or it may be modular, where you can choose how many chambers you want – and it may come with or without filter media. It's also possible to make your own koi pond filter, which is cheaper, but a lot more work.

Koi pond filtration – mechanical and biological

Koi pond filters do two kinds of filtration: mechanical and biological. Both are vital for the health of your koi.

Mechanical filtration

3-chamber filter

Koi produce a fair bit of solid waste, and it needs to be filtered out of the water so that it doesn't poison your koi. Your koi pond filter system can use various types of filter media for mechanical filtration: for example, nylon brushes, or plastic media. It can also use a water vortex that separates out particles by centrifugal force. The vortex is the only kind of mechanical koi pond filter that doesn't need regular cleaning, and it's also probably the most efficient. One good approach for your filter system is to have a vortex filter followed by a chamber with brush filter. You can add more kinds of filter if you have space, time and money – better too much than too little. More about koi pond filter media below.

Biological filtration

The biological filter chambers use friendly bacteria to process the ammonia which koi produce in their waste. The bacteria turn the ammonia into nitrite and then nitrate, and that can be kept in check with your usual weekly water change (see koi pond maintenance). For biological filtration, you'll need chambers which contain some suitable material for the bacteria to live on. Most mechanical filter media also provide good biological filtration (probably the best material for a biological filter is Japanese matting, which allows the water to flow freely, and doesn't clog) but you can also get specialist biological filters.

After you build your pond, it will take a month or two for the bacteria in your filter to multiply to the right levels, so until that's happened, it's best not to introduce too many koi, or feed them too much. (In fact, it's a good idea to increase the number of koi in your pond gradually, a few at a time, to give the filter time to adjust.)

Biological filtration will also help stop algae growing in your koi pond, so you may find that after the pond is built, the pond goes quite green, but this should improve once the bacteria are established in your koi pond filter. Don't be tempted to try and kill off the algae chemically, as this will be counter-productive in the long run, as it will stop the bacteria growing.

If you want to speed the process up, you can get dried bacteria culture which you can add to your filter.

Bacteria will also be more active in summer than in winter, so you may get an algal bloom in winter, too – but again, don't be tempted to treat it chemically, as this will damage the bacteria in your filter.

Types of koi pond filter system

There are basically two kinds of koi pond filter system: pump-fed filter systems and gravity-fed filter systems.

A pump-fed filter system has a pump submerged in the pond, which pumps water from the pond into the filter. The filter sits above the ground. The clean water pours from the filter back into the pond.

A gravity-fed filter system works by gravity rather than pumping. In this case you have drains at the bottom of your pond, and gravity pushes the debris down through the drain, along pipes, and into the filter. In this case the filter needs to be buried so its top is level with the ground. That's because gravity will ensure that the level of the water in the filter is the same as the level of the water in the pond, so if one is higher than the other, you'll get overflow. Gravity-fed filter systems also need a valve so you can close the system off from the pond for cleaning.

Koi pond filter media

• Brushes: nylon brushes are ones of the cheapest mechanical filter media.
• Filter material is honeycombed sheets which do double duty for mechanical and biological filtration. Japanese matting is a great one – it allows a free flow of water.
• Ceramic biological filter media offer great biological filtration, as they have a huge surface area for the bacteria to live on. Can be quite expensive, though, as they're mostly imported from Japan.
• Plastic media: there are a wide range of plastic filter media out there, for both mechanical and biological filtration. They're generally in the form of some kind of little chip shaped to increase the surface area.

You can get bags and mesh trays to hold your filter media in the chamber.

Other koi pond filtration methods

Vortex filters

Old style filter systems work by passing the water through filter media which sieve out the debris. Vortex filters work quite differently. They spin the water, and the centrifugal force sends all the waste in the water to the sides, where they hit the walls and drop down to the bottom of the chamber. The waste can then be removed just by opening a valve at the bottom.

Vortex filters are probably the most effective filter available – they can remove over 75% of the waste that passes through them. They make a great first chamber in a multi-chamber filter system.

Protein skimmers

These are marvellous things which dramatically improve water clarity and colour by removing proteins and very fine solid waste from the water. They're especially useful if you have a problem with foam on the top of your pond, as this is caused by proteins and other dissolved compounds.

Basically they work by frothing up the water inside the skimmer; the proteins etc stick to the bubbles, and this foam is passed to the waste out pipe. The remaining water is returned to the pond, much cleaner and clearer than it started. As an added benefit, all the foaming raises the oxygen level of the water.

Trickle filters

Trickle filter

Trickle filters, also called trickle towers, are a very effective kind of biological filtration. They work by pumping water to the top of the tower, then spraying it over the filter media. The water trickles down through the media, and back into the pond. The reason they're so effective is that the bacteria on the filter media are not underwater, they just have water pouring over them – this means they have more oxygen, which means they process much more ammonia, and can even turn nitrates into nitrogen gas.

Pressure bead filters

These contain small beads, under pressure. They offer both biological and mechanical filtration.

Choosing your koi pond filter

If you're building a large pond, and you're serious about koi keeping, in my opinion, it's worth installing a gravity-fed filter. You need to decide this before you start planning your pond, as if you're going to use a gravity-fed filter, you need bottom drains, and the pipes to carry the water from the bottom drains to the filter – and obviously that all has to be laid before you start pouring concrete!

Whatever approach you're taking, when choosing your koi pond filter, you're going to need to think about the size of your pond, and the number of fish you plan to have. You need to be able to pass the entire volume of your pond through the filter once every two or three hours, so the larger the pond, the bigger a filter you need. And the more koi you have, the more waste they produce, and again, the bigger a filter you need. Commercial filters will tell you what size pond and how many koi they're designed for. If in doubt, get something bigger than you think you need, rather than risk getting a filter which is too small.

Think also about how much time you have. Some koi pond filters require more maintenance than others. So if you're short on time, go for something which is easier to look after, such as a pressure type bead. This will be more expensive, but will take up less of your time. They do need backwashing occasionally (see koi pond maintenance), but this will only take a few minutes, and isn't a messy job, just a question of turning a lever.

See also koi pond construction.

Filter photos by mylifestory.