These days its not hard to find a reason to save a few bucks. Building your own koi pond and filtration system can save you thousands of dollars, plus it comes with the pride of having built it with your own two hands. There are two ways to go about a DIY koi pond. One is to buy all the filtration equipment and simply install it yourself, or build your own filter from scratch. Commercially produced filters will generally be far easier to install and require less time to maintain. But for the true koi pond tinkerer, building your own filter is far more fun and rewarding.
Before you decide to build your own filtration, first you need to understand the basics of mechanical and biological filtration in koi ponds. In almost all cases mechanical filtration will be installed before your biological filter. This will remove solid waste from the water before it enters the bio section. Sometimes this can be done in a single filter, but its more effective if you have two separate filters, one for mechanical and one for bio. The easiest type of mechanical filter to build will be a settling chamber. For more information on settling chambers and their use in a koi pond, read this article.
But there are other types of mechanical filters you can install in your koi pond, most of which are cousins to the settling chamber. The best DIY method will be gravity fed. The main reason is a pump fed system will puree the koi waste and make it much harder to remove. An excellent gravity fed mechanical filter you can purchase is the Cetus Sieve. It uses a microscreen to remove solid waste. It is installed in the same manner as a settling chamber, only it can handle a much higher flow with a much smaller footprint. Another type would be a chamber that has filter brushes. Filter brushes trap loads of debris, almost never clog and they are easy to clean. Generally you would need to build a separate gravity fed section near the koi pond and hang the brushes filling the entire chamber. This can be a separate liner pond, a 55 gallon barrel or even a cooler/container with bulkheads installed. Mechanical chambers can also be filled with things like gravel and sand, but I have found these difficult to get clean and not as effective for mechanical filters. Regardless of what type of mechanical filter you install, make sure it is easy to clean and drain. Always install a bottom drain with a valve that can be opened and the waste flushed away.
Once you have decide which type of mechanical filtration you want in your koi pond, the next step is to design your biological filter. For more information on how a biological filter in a koi pond works, please read this article. There are several ways you can go about building your bio filter. It can be gravity fed or pump fed. A pump fed bio filter will pull clean water from the mechanical filter and send it to the bio filter. A gravity fed filter will be in ground, at water level and attached to the mechanical filter with some type of overflow system. Then the pump would pull water from the bio filter and send it back to the koi pond.
There are endless ways to design a bio filter. Many popular designs will use 55 gallon barrels. They are easy to get, cheap, hold a good amount of water, easy to cut and install bulk heads and in general make for a perfect bio filter for koi ponds. But any sort of container will work. Rubbermaid makes some good quality, sturdy containers as well. A popular method for a bio filter in a koi pond is the up flow. The up flow has the water coming up through the bottom of the container, passing through your bio media and then overflowing back to the koi pond. The benefit if this type is its easier to hide. However my preferred style of filter is the down flow or shower filter. The requires the chamber be elevated above the koi pond making if more difficult to hide, however it is a far more effective type of filter. Basically you will rig a spray bar over the top of the chamber. The pump sends water up and showers it over the media. The water trickles down through your media and back to the koi pond. This creates an oxygen rich environment which is ideal for growing bacteria. If you don't want to elevate the chamber completely over the koi pond, you can have the return line leave at the bottom of the chamber but then immediately elbow up before returning back to the pond. Then the chamber will fill with water up to the level of the pipe. Doing this will allow you to have at least ½ the height of the chamber below the water return, making it at least a little easier to hide.
The type of media you use is up to you. Remember, its all about surface area. The more surface are you can cram into you bio filter the better. I have seen all sorts of things used. Hair curlers, plastic army men, scrubby pads, bird netting, etc. Go hit the 99 cent store and look for anything that will give you lots of surface area. One of my favorites is poly strapping material, the type they use for tying down pallets. You can get a 9000 ft roll of ½ in strapping for less then $50. And it just so happens a 9000 ft roll will perfectly fill a 55 gallon barrel.
I generally try to avoid gravel and lava rock in filters. They are very heavy and difficult to clean. If you do go this route, be sure to install two things. One is a way to drain the gravel chamber. Second is an air line along the bottom of the chamber. Basically just criss cross 1/2” pvc along the bottom with small holes or air stones attached. This is powered by a spa blower. When you want to clean the gravel, you turn on the blower and the rising air breaks all the muck and gunk free. Then drain it away. If you do not install both the drain and the air lines, a gravel filter will be very difficult to maintain.
Whatever style of filter you decide to build, always keep in mind the best thing you can do for yourself and your koi is to make it easy to clean. If the filter is difficult to clean, well lets be honest, it simply won't get done as often as it should. Install drain valves on every chamber and use lightweight plastic media so it can removed without a lot of effort.