What a Koi needs - they are very demanding fish
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Why Koi Need a Lot Of Filtration
A few thousand years ago, Goldfish decended from Crucian Carp which naturally lived in ponds. The chinese kept them as a food fish, and the gold ones started appearing. Being a novelty, the Chinese kept the gold ones, and since they could make large ceramic bowls, they started keeping the fancy gold ones in their homes. The goldfish would spawn in the bowls, the ones that could take the poor water conditions survived, and today we have a very hardy fish which can live in horrible conditions. Since keeping them was so easy, they are known as "the people's fish".

Koi decended from the common carp which evolved in a river environment. They have been kept in ponds as a food fish for about the past thousand years. Since they needed a pond, the only people that could keep them were either village ponds or the upper class in large estate ponds. So they haven't had the selection process like goldfish have to be able to withstand poor water conditions.

Koi also grow very fast, typical koi get to 12" within 1 year, 20" within a 2 years, and 30" within 10 years. The japanese special breed jumbo koi get to be 40" and 25 lbs+. So you can see how they need a lot of room and lots of filtration. Poor water quality and cramped conditions tend to stunt a koi's growth, causes health problems and fatalities.

Pond Size
You need a minimum of 1000 gallons for the first koi, then 300 gallons for each additional. This might seem like a lot of water when you look at a little 12" or 18" koi, but you have to keep in mind that the koi is going to continue growing and it will be very big in just a couple of years. Serious koi enthusiasts have 1000 gallons of water per fish. The average pond in our club seems to be between 3500 and 5000 gallons, but many are much larger than that, like around 15k - 30k gallons. To figure how big your pond is, figure how many cubic feet, then multiply by 7.5 gallons per cubic foot. Or you can empty your pond, then read your water meter. Fill your pond, and read your meter again. (make sure to dechlorinate and acclimate your fish before returning them to the pond)

Pond Shape and Depth
The two main predators that attack koi are Herons and Raccoons. They both hunt in similar ways, they wander into the pond in the shallow end, and then wait for a fish to come close, and then strike. One of the best defenses against this is to have a pond with walls that go straight down for 24" or more. As for overall depth, koi like deep water, like in the 6' to 8' range. This is not practical for most casual pond owners, so please consider atleast 24" or greater depth.

Running Current
Koi decended from river carp, and love to have a constant current to swim in. The closer you can make your pond a circle or oval, the better they will like it. Also have your water returns go into the pond in a way so that it creates a current.

Pump & Flow Rate
The minimum actual flow rate you should have is half the pond volume, per hour. So if you have a 1000 gallon pond, your actual flow rate should be atleast 500 gallons per hour. It is much better to have a flow rate which will turn your pond over 3 times per hour, so with that same 1000 gallon pond, your actual flow rate would be 3000 gallons per hour. The high flow rates increase the effectiveness of biofilters and help aeration. ** You need to measure the actual flow rate . Pumps come with a rated flow rate, and after you pump the water thru a pipe, the actual amount of water coming out is much less than what the rating is. Very simple to test it, just time how long it takes to fill a 5 gallon bucket, and calculate from there.

How a Biofilter Works
A pond filter works VERY DIFFERENTLY than a pool filter does. What is going on is that koi eat food, then they produce waste which is organic matter plus ammonia. There is a type of bacteria that eats ammonia, and converts it into Nitrite. There is another type of bacteria that eats nitrite, and converts it into Nitrate. Algae & plants eat the nitrate. And finally the koi eat the algae and plants, starting the cycle over again.

The bacteria described above grows on the surface of everything in your pond, the only problem being that the pond walls and pipes in your pond are not big enough to have enough area, to grow the amount of bacteria to process your waste products. So chambers full of "media" is put inline with the circulating pump, and the bacteria grows on the media. The media could be lava rocks, pea gravel, bio balls, pvc shavings, pot scrubbers, bubble beads, or whatever - the point is to make a large surface area for the bacteria to grow on. As the water is pumped thru the media, the bacteria can feed on the waste products and do their conversions.

To get rid of the free floating organic matter, you can put a settling chamber inline with your pump before the media, so the big particles will settle out. There are many different types of settling chambers, the most common being a simple vortex chamber.
For an ugly, but functioning simple filter see [ The ShortyPen Barrel Filter ]
There are many, many different types of biofilters and configurations.

Sizing your Filter
Everything in a pond is related to the weight of the fish, not the water volume. I see many filters in stores and online that say they are rated for XX gallons of water, when infact they are actually processing machines and they should be rated for a certain amount of food per day. A good rule of thumb is that you need 10 gallons of media for each koi. If you have 5 koi, then you need 50 gallons of media. Some media is more effecient than others, so you might need more of one and less of another.

Settling chambers often work good with a 5 minute dwell time, so you figure how much water your pump moves in 5 minutes, and that is the size in gallons of your settling chamber.

Algae
There are many types of algae, and it is GOOD for your pond - the koi eat algae, and it helps the biofiltration. The problem is with the free floating "pea green soup" algae, it blocks your view of the fish which you spent all this time and effort to have. The best way I have found to get rid of the green soup algae is to setup a UV sterilizer with it's own pump. That is basically a light bulb inside a tube, and as you push the water past the light bulb, the UV light rays kill everything. The pump needs to be sized with the UV unit so it can run the water through at the correct flow rate, so it is slow enough to allow the UV light rays to kill everything. After a week or so, the free floating algae will clear out and other types of algae will start coating the walls of your pond. Those other types of algae will then start consuming the nitrate, and out-compete the free floating algae so it won't come back as easily. You can turn the UV & it's pump off when the water clears up, or you can leave it running all the time because the UV sterilizer also kills certain types of bacteria and other stuff which may help the health of your fish. UV bulbs usually only last about a year and have to be replace.

Water Changes
You should remove a minimum 10% of the pond water volume per week, and replace it with new fresh water. A few reasons for this, first it gets rid of excess nitrate and other byproducts. But mostly becasue of the evaporation process. So much water evaporates from our ponds, but that leaves all sorts of dissolved stuff behind. If you only "top off" your water level, then the dissolved solids build up over time and ruins the water quality. Keep in mind that koi came from rivers, which are continually supplied with fresh rain water.

My pond water is clear, doesn't that mean it is healty water?
No. Clarity has nothing to do with the quality of the water. Ammonia, nitrite, and nitrate are all toxic to koi. You can have perfectly clear water, and kill your fish, or significantly stunt their growth. To see what levels you have, you need to test your water with test kits.


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