Koi Filtration

A Proven Filter System

Setting up a koi pond filtration system can be a complicated endeavor with a lot of guesswork, unknowns and countless hours of research.

Here is a run down of a tried tested and proven filtration system. It’s constructed of barrels which are inexpensive and will out-perform manufactured filters that cost thousands of dollars. I will be adding videos of each of the components with how to instructions and information on their functions.

Subject links To This Page 
Trickle Tower
Vegitation Filter
De Chlorinator

If your water comes from a municipal source it will have toxic chlorine or chloramines added to it to protect humans from bacteria and parasites. Chlorine can be removed by evaporation but chloramines must be removed chemically. You can contact your water municipality and ask which they use. Either way the water will need to be treated before entering the pond.

I use three 55 gallon drums at a height greater than the ponds surface.  They are individually plumbed with fill valves from the water source. Then they are individually plumbed with dump valves to release them into the pond.

One drum can be dumped into the pond after treatment and then refilled. A few days later the second drum is dumped and refilled followed by the third a few days later again. By the time you get back to the first drum it has been sitting, dechlorinating and reaching ambient temperature for 9 days.

If your water source uses chloramines the same process can be used except you will need to add a water conditioner such as Amquel that removes chloramines.

Bottom Drain

If there is any possible way to make a gravity bottom drain work in your pond construction location, I would go through whatever hassle or extra expense it takes to install one.

A bottom drain is essential when it comes to pond maintenance which is greatly reduced by a gravity bottom drains ability to feed a gravity filtration system.

Bottom drains need to be placed in a location that uses the ponds circulation to sweep everything into it. Depending on the pond size shape and depth layout, it may take more than one bottom drain to properly do the job. The benefit of a gravity feed system is that you can filter out the solids before they go through the pump.

If a gravity bottom drain is just simply not possible, a retrofit kit is available for a bottom drain to work from a discharge located higher up or even over the edge. While the retrofit drain setup to go all the way above the pond surface and over the ponds edge will (if properly placed) still clean the bottom of the pond, it requires an inline pump to pull the water out which grinds the solids into fines rendering the gravity fed filters pointless.

Gravity Feed

A gravity feed system can be a bit difficult to construct but I think it’s well worth the effort.

A gravity feed sounds simple in theory, it’s just a filter sunk down to the pond’s surface level so that gravity fills it without the need to run it through a pump to get it to the filter. The obvious advantage is the elimination of the pump which grinds everything into a waterborne, difficult to remove, dust.

What makes a gravity filter difficult to design is what I call, “line loss”. Line loss is the volume of water and it’s momentum that you loose with every inch of pipe it travels down and even more so every 90 degree turn it makes and “T” that it has to pass through.

People get really out there with scientific formulas and complex mathematical equations to design these: Scroll through "this page" right quick!

These give you a starting point but basically just oversize all of the plumbing and minimize twists and turns in the path the water must travel. 


Settling Chamber

A settling chamber is the first step in a filter system and incredibly efficient in a gravity system. It’s simply a chamber where the heavy solids have the opportunity to fall out of the current and settle to the bottom where it lies in wait for somebody to open the drain valve and shoot it down the waste drain.

I like a “vortex” system which is a round settling chamber with the water flow entering in such a way as to swirl the water in a circular motion with the exit near the top and in the center. It,s efficiency amazes me.

A Settling chamber has to be designed so that the water speed and chamber size work in combination to be effective. It is very difficult to get the proper flow and many people fail in their construction.

I've discovered a little secret trick that easily eliminates this problem. If you purposely oversize everything for too much flow, you can slow down the speed to the perfect rate by hanging filter brushes in the tank which has the added bonus of further eliminating the fines.


A skimmer effectively pulls the floating debris from the surface of the pond before it has the opportunity to sink to the bottom. Large debris like leaves and such are caught in a basket or net while the fines, including dust that can otherwise coat the ponds surface, continue on to the filtration system.

I run my skimmer on a separate line to the gravity system after bypassing the settling chamber. This allows it to be operated on the same pump without further grinding up the fines or adding floaters to the settling chamber.

The skimmer is built into the edge of the pond where a floating weir allows it to pull water just off of the surface regardless of water height fluctuations caused by evaporation or water changes.

Mechanical Filters

A mechanical filter is a chamber with some form of mediawhich the water has to pass through which captures the light weight particles. Screens, brushes and mats are all considered mechanical filtration. So long as you don't clean these with chlorinated water or let them dry out, they will also act as an additional bio-filter.



Matala ~ Matala is a brand name filter media that comes in different texture thicknesses. It can be purchased in a sheet form or rolled into a disk. The disks work well wedged into a round filter chamber and can be placed from the coarsest to the finest. They work in an up-flow or a down flow chamber.

I use them lie bags of pond additives on such as oyster shells, crushed coral or zeolite.

Matala does an excellent job of grabbing onto the tiniest waterborne particles, it’s almost as if its statically charged. The greatest thing about it is how easy it is to clean.


Now that the water is clean, Unless you’ve invented perpetual motion, it has to at some point be pumped in order to get it back to the pond.

The pump is the heart of the system. If the pump fails the oxygen depletes then the bacteria begin to die then the ammonia, nitrites, and nitrates climb then real problems begin.

I like to use two pumps so there is always a back up. A secondary pump could be a repeat of the main pump or a smaller version that switches with the main pump at night. It could even be a pump that runs a fountain or UV light as long as it’s plumbed so it can run the filtration system even if it’s just barely in order to keep the good bacteria alive and the water aerated.

Pump sizing depends on a lot of factors such as fish load, pond size, water features, Etc. but a good general rule for a koi pond is enough GPM (gallons per minute) to turn over the pond every hour.


Pumps have greatly improved over the days of old, with magnetic drives and designs intended for continuous operation they are much less expensive to operate.


When sizing a pump, remember to check the “head” pressure flow rate. Pumps are rated fat zero feet of lift. For every foot the water has to be pumped up (head) there is a loss of flow rate. Different pumps will all have different head rates.


UV Filters

If you have an algae bloom, the fish are in no danger but you could end up not seeing your pretty little pets for quite some time and the pond doesn‘t look so great either.

A UV light is the answer for this. It needs to be properly sized for your ponds gallons and the flow pumped through it has to be at the proper rate. It is possible to use your existing pump even if it’s to fast by installing the UV light on a bypass valve with a flow meter. In the end though it may be less expensive to just install another pump which could also act as an emergency back up pump should your main pump fail.

A UV light for the purpose of clearing algae blooms is generally about 10 watts per 1000 gallons of pond water.

Another option is to go big which gives you a “UV Sterilizer”. What is the difference between a UV light and a UV sterilizer? Well it gets a bit tech-y with a lot of information on wavelengths, mercury pressures and the like but essentially it is the power output. A UV sterilizer has all the right stuff to kill microorganisms whereby sterilizing your water from a multitude of bad guys. The bad guys will have to be free-floating in the water in order to pass through the light.

One last note on UV Filters. The bulbs loose efficiency before they burn out and need to be replaced according to the particular model.

Bio Filters

A bio-filter is a must in any pond stocked with koi. A bio-filter is where the “Nitrification Cycle” is established and the good bacteria perform their critical function.

It is a chamber full of media through which the pond water circulates. Anaerobic (meaning they need oxygen) Microorganisms, grow and colonize on the media which transform the toxic ammonia and nitrite into nitrate. For more detail on this see: “Nitrification Cycle”.



I’ve read a lot of controversy about where to locate the bio-filter in relation to theUV filter.

My logic was this:

The mechanical filters have a good deal of bio-filter media before the UV filter.

The UV Filter kills some of what gets dislodged and waterborne as well as any other bad guys before they end up in the main bio-filter.

The good guys in the main bio-filter that get dislodged end up all over the pond walls and plumbing with maximum opportunity to grab onto something before they go through the UV filter.

My UV filter is a sterilizer. It is before the bio-filter and everything works excellent and creates perfect water parameters.



What is Bio-Media?

Bio-Media ~ It’s all about surface area. The more surface area, the more room for bacteria to colonize. The more bacteria, the more efficiently the water is treated.

Porous material such as lava rock or crushed coral have huge surface area and work well for growing good bacteria however they can be difficult to clean which has lead to a vast range of man made bio-media. It is based on square feet of surface area per. cubic foot of bio-media.