Koi Pond Filtration Basics

In any good Koi Pond we need two different kinds of filtration:

Biological Filtration (ESSENTIAL to understand for all livestock setups)  

Biological Filtration is the breakdown 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 performed 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 the 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. Basically the less water, and more oxygen the better. 

Trickle Towers or Showers:

The open tray Trickle Towers or Showers are awesome for biological filtration. 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 Bottom Drain circuit. Bottom Drain water is lower in oxygen than surface skimmer water. This low oxygen water is where we want to put our oxygen enhancing biofilters.

In most cases it is usually best to gravity flow, or “suck” the water through the biofilter. The best way is to have your biofilter in line after your settling tank, so your pump is pulling water through the Bottom Drain, through the settlement tank, through the biofilter, then to the pump suction. The one exception are trickle towers and showers that have to be pump fed.

Bead & Sand Filtration 

Bead & Sand Filtration (aka.Fines Filtration) removes the fines that cloud the water. This is what if done properly, will give us the “Gin Clear” water we all want. Most mechanical Filtration is pump fed, such as Bead Filters and Sand/Gravel filters. Basically, the mechanical 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.

And Now for Mechanical Filtration: 

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 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 Bottom Drains will handle different size ponds. For the Bottom Drain to be effective, the flow rate has to match the Bottom Drain pipe size. If the flow rate is too slow, sediment will settle out in the Bottom drain 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 Bottom Drain to give enough “Dwell Time” to allow the heavier than water solids to settle out. In general terms, the settlement chamber should be no less than 10% of the flow rate.

A three inch (3") Bottom Drain will effectively sweep a four foot (4') radius and 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 (4") Bottom Drain will effectively sweep a six foot (6') radius, a 4-inch bottom drain needs a minimum flow of 2500 gph to prevent sediment from settling out in the line. This 2500 gph needs a minimum of 250 gallon (10%) settlement tank to effectively settle out the heavier than water solids. Note: a 3500 gph/350 gallon settlement tank is better and preferred for a 4-inch Bottom drain if you have the room. 

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 Bottom Drain into the settling tank to fill the void. Pumps should never be hooked directly to a Bottom drain, 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. The settlement tank has to be buried in the ground, or in some kind of filter pit as water seeks its own level, the Bottom Drain 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 countercurrent of water flowing down along the outer pond walls, then across the bottom to the Bottom Drain, thus sweeping the bottom clean even better.

Bottom Drain performance can also be enhanced with underwater 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 Bottom Drains.

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.

Sieves: 

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 more costly to buy.

Skimmers:

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. Without a skimmer the pond surface will collect a layer of Dissolved Organic Compounds, 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 inside a building.

Bead & Sand Filtration 

Bead & Sand Filtration (aka.Fines Filtration) removes the fines that cloud the water. This is what if done properly, will give us the “Gin Clear” water we all want. Most mechanical Filtration is pump fed, such as Bead Filters and Sand/Gravel filters. Basically, the mechanical 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.

Matting Filtration

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.

Ultraviolet Sterilization

Commonly called “UVs”. As our pond water gets warmed and exposed to more and more sun we will start growing a microorganism or “Algae” that will turn out water green. The only way we can kill 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. Play it Koi can help you find the right size of UV for your setup.