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Author: Steve Joneli - High Desert Koi

Filtration Basics
In any good Koi Pond we need two different kinds of filtration. Mechanical, for us humans to keep the water clean and clear for our viewing pleasure, and biological to culture the Aerobic Bacteria that break down and consume the waste the fish produce. (Ammonia). Let’s start with Mechanical.

Bottom Drains
Our ponds will produce heavier than water solids, mostly fish waste, that being heavier than water will settle on the pond bottom. We use what we call “Bottom Drains” (BDs) to sweep the bottom clean of this heavier than water solids. Bottom Drains are designed to pull from the circumference or sides of the drain, instead of straight down through an open hole, thus sweeping the bottom clean around the Bottom Drain. Bottom Drains usually flow into either a settlement filter, allowing the heavier than water solids to settle out, or into a Sieve, which separates the heavier than water solids from the main water stream. Different size BDs will handle different size ponds. For the BD to be effective, the flow rate has to match the BD pipe size. If the flow rate is to slow, sediment will settle out in the BD piping. The more you flow, the larger your sweep radius will be. And the size of the settlement chamber has to match the flow rate of the BD to give enough “Dwell Time” to allow the heavier than water solids to settle out. In general terms the settlement chamber should be 10% of the flow rate.
A three inch BD will effectively sweep a 4 foot radius, a three inch bottom drain needs a minimum flow of 1500 gallons per hour, (gph) to prevent sediment from settling out in the line. This 1500 gph needs a 150 gallon (10%) settlement tank to effectively settle out the heavier than water solids. 2500 gph/250 gallon tank is better and preferred for a three inch BD.
A four inch BD will effectively sweep a 6 foot radius, a four inch bottom drain needs a minimum flow of 2500 gallons per hour, (gph) to prevent sediment from settling out in the line. This 2500 gph needs a 250 gallon (10%) settlement tank to effectively settle out the heavier than water solids. 3500 gph/350 gallon tank is better and preferred for a 4 inch BD.
The line from the bottom drain flowing into the settlement chamber is what we call gravity flow. The pump does not take suction off this line, but takes suction off the settlement tank, thus dropping the level of the settling tank, As this level becomes lower than the pond, water from the pond will flow through the BD into the settling tank to fill the void. Pumps should never be hooked directly to a BD, as the heavier than water solids will get minced up going through the pump impeller and become almost impossible to separate out. This is why we use settling tanks before the pump. As you can see, the settlement tank has to be buried in ground, or in some kind of filter pit as water seeks its own level, the BD will always try and keep the settlement tank filled to the pond level.
Bottom Drain effectiveness can be enhanced by adding an air diffuser to the top of it. Most air diffusers for this application are 9 inches in diameter. The column or air rising to the surface will carry a column of water with it. This rising water column in the center of the pond will create a counter current of water flowing down along the outer pond walls, then across the bottom to the BD, thus sweeping the bottom clean even better.
Bottom Drain performance can also be enhanced with under water returns commonly called TPRs or GPRs. These are under water returns, usually about 16 inches off the bottom of the pond that will direct the bottom currents in a circular flow around the BD, thus freeing settled waste so it can be carried away down the BD.
Settling chamber size can be reduced with the aid of a Pre-Filter or rotating Micro Screen on the outlet of the settling chamber. The one problem with settling chambers is they do not remove the fish waste from the water column, but just collect it in wait for us humans to either dump it to waste, or pump it out to waste. We humans have to do something to physically remove the solids the settlement chamber has collected. Until we do this, the fish waste we have collected is still in the water column polluting the pond.
A Sieve in place of a settling tank will actually physically separate and remove the solid waste from the water column and hold it in a separate compartment awaiting removal by us humans. Sieves are smaller than settling chambers, but harder to construct our selves (DIY) and more costly to buy.

Skimmers remove floating stuff like leaves, pine needles and so forth. They also remove suspended and floating “Fines” that tend to cloud out water. Also and most important, the skimmer will remove the layer of Dissolved Organic Compounds (DOCs) that form on the pond surface. With out a skimmer the pond surface will collect a layer of DOCs, it will look like an oily film floating on the surface, effectively sealing the pond surface and suffocating the pond. This would be just like laying a piece of Visqueen over the pond surface.
The opening of the skimmer will have a weir, a floating device designed to float up and down with water level, thus skimming only the surface. This weir will also aid in keeping fish out of the skimmer. Always go with the widest weir available. A 16 inch weir will skim twice as much water as an 8 inch weir at the same flow rate. Any good skimmer will have some kind of removable “Leaf Basket”, that will trap lg stuff like leaves. You can remove this basket and dump and hose it out. The rigid solid poly leaf baskets are easier to use than the ones made with netting. Some skimmers will provide additional mechanical filtration usually in the form of matts or brushes. The kind of skimmer you need will depend on your pond’s environment. A dirty pond, one that has lots of leaves and other floating debris will need a larger weir and leaf basket than a clean pond, such as one that is inside a building.

Biological Filtration
Biological Filtration is the break down and consumption of the ammonia the fish waste produce. Without biological filtration the ammonia and nitrite levels will rise to the point of killing the fish. Biological filtration is preformed by Aerobic Bacteria, what we lovingly call “Bugs”. This bacteria will consume the waste ammonia the fish produce and break it down to Nitrites, then a 2nd kind of aerobic bacteria will consume the nitrites and break that down to Nitrates. In low levels Nitrates are harmless to the fish and can be controlled by water changes.
The Aerobic Bacteria we want to grow is oxygen loving (aerobic) It has to have some place with lots of surface area to grow on, in an oxygen rich environment. So what we want is some kind of container with some kind of media with hundreds of sq feet of surface area, with oxygen added to culture the bacteria. There are dozens of DIY and store bought filters that can accomplish this. But basically the less water, and more oxygen the better. An enclosed, pump fed filter is the worse kind of bio filter there is, an open top filter, such as a barrel full of some kind of media is better. Add an oxygen supply to this barrel filter with an air diffuser on the bottom powered by an air pump is even better. The open tray Trickle Towers or Showers are the best. They are what we call a dry environment, not submerged under water, but dry stacked so to speak, with water flowing down over the media.
In most cases Bio Filters should go on the BD circuit. BD water is lower in oxygen than surface skimmer water. This low oxygen water is where we want to put our oxygen enhancing bio filters.
In most cases it is usually best to gravity flow, or “suck” the water through the bio filter. The best way is to have your bio filter in line after your settling tank, so your pump is pulling water through the BD, through the settlement tank, through the bio filter, then to the pump suction. The one exception are trickle towers and showers that have to be pump fed.

Fines Filtration
Fines Filtration is just that, it removes the fines that cloud the water. This is what if done properly, will give us the “Gin Clear” water we all want. Most mech. Filtration is pump fed, such as Bead Filters and Sand/Gravel filters. Basically the mech. Filter will be full of some kind of fine filtering media like poly beads or course sand. This media will trap and hold the suspended fines until we humans flush the waste out and clean the filter. Another kind of mechanical filter is Japanese Matting. The water is drawn or sucked through a chamber with progressively finer and finer rows of Japanese matting, thus trapping the fines. Like pump fed filters, this matting has to be cleaned from time to time. Any mech. Filter will lose its effectiveness and eventually plug up if not cleaned often enough.
We humans tend to be lazy, letting things go until we’re forced to deal with it. Our pond cleaning chores will be much easier and take less time if done on a daily basis then a weekly or monthly basis.

The heart and sole of our ponds. Everything living in our ponds depends on good water circulation. Our pumps need to run flawlessly 24/7, year after year. Pump selection and sizing is very critical for optimum efficiency. More on this later.
Generally speaking an out of the water centrifugal pump is cheaper to buy and cheaper to run than an in water submersible pump. Submersible in water pumps tend to be costly to buy and power hogs to run, in some cases drawing 10 times as much electricity as a same sized centrifugal pump. Plus there is the chance of an electrical short and electrocuting your pond. There are far to many things to consider when choosing a pump, such as flow rates, head height, self priming or not, and so on. This on one place where you want to find a dealer you trust, and follow his or her recommendations.
Suction lines to pumps should always be one and one half times the pump inlet. So if your pump suction inlet is 2” the line running say from the skimmer to the pump should be 3”. At the pump suction, neck the 3” down to 2” going into the pump. This is a must if the pump will be located above water level in a self priming configuration. Pumps should always have isolation valves on the inlet and outlet for blocking in the pump to service the priming pot/strainer basket.

Ultraviolet Sterilization
Commonly called “UVs”. As our pond water gets warmed and exposed to more and more sun we will start growing a micro organism or “Algae” that will turn out water green. The only way we can kill this green algae is with a germicidal bulb or UV. This bulb will emit ultraviolet radiation, effectively killing everything that flows past it. The UV sterilizer should be placed after the pump and filtration for best results. The size of the UV sterilizer needed is dependent on flow rate and pond size. Once again, here’s where a trusted dealer can help you choose what will work the best for you.

Knife valves have a flat blade or paddle in them. These valves are only made to be either fully open or fully closed. Not part way for flow control. In larges sizes such as 3 and 4 inch, the knife valves work great in the BD lines for blocking flow while servicing the settlement tanks.
Ball valves are used for flow control and isolation. They can be run part way open or closed. Stay away from the so called economy PVC ball valves. Over time they will get difficult to operate, eventually breaking. Use a good quality single union or true union ball valve.


To answer some questions I like the Bottom Drain water to gravity flow to either a sieve or settlement chamber or vortex. If settlement chamber, put some kind of pre-filter or micro screen in it. Then gravity flow to the biological filters, then pump suction, then fines/mech filtration, then TPRs.
If you put your primary biological filtration on your BD circuit, you then can send Skimmer water straight to the pump, then to fines/mech. filtration and out the waterfall.

In Closing
Remember, the overall success of our ponds will only be as good as it’s weakest link. And the amount of maintenance we humans provide. If in doubt about equipment or design, ask questions, get advice. You don’t want to have to build this twice.

By DreamPond Admin


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