Filtration, Circulation, Aeration and Cleaning

Published on September 2, 2014

An ADDR air-driven dilution reactor.
An ADDR air-driven dilution reactor.

Four main factors in building successful koi ponds are filtration, circulation, aeration and cleaning. In future articles we will discuss each of
these in-depth, but initially we should look at how these relate from an overall perspective when designing a system. A koi pond is a decorative wastewater treatment plant that takes care of the needs of the fish by dealing with the fish’s waste in real time. Good filtration, circulation and aeration are what
you design into the system for the fish. The ability to easily clean and
maintain the system is what you design in for you or your customer.

What is this Pond For?

The first consideration in the design is the purpose of the pond. What is the customer’s intent for the feature? A koi pond has the highest turnover rate and filtration demand and usually needs a turnover rate of at least once per hour.
What is the intended fish load? Be careful here, because customers will generally mislead you and overstock their pond once you’re gone, so plan for this. Fishkeeping is not a hobby, it’s a disease, so start out slowly, leaving room for additional fish in the future. Placing a large number of fish immediately into a new system is a difficult way to initiate the start-up phase of a pond. Start with just a few fish and let the system “kick,” establishing
the bacterial colonies, initially stabilizing the system to “zero” ammonia and “zero” nitrites before adding more fish.

Atlantic Oase Reward Program

After the size and volume of the pond you’re building have been established, you can calculate the total flow in pumped water volume necessary to move all the water through filtration in one hour or less. It is important to flow all the water through filtration whenever possible with 5 to 10 percent added for filtration volume.

With a few exceptions, circulating water out of and back into the pond without doing any bioconversion is a waste of energy. Water flowing down a stream adds oxygen and can do some bioconversion, and water from air lifts can circulate and oxygenate the system. There are other examples, but you must
be careful about how much you count on these to do any bioconversion.

An air-lift current jet in operation.
An air-lift current jet in operation.


Once you’ve established the total flow rate, you can determine how water will
exit from the pond and then determine how it will return. Pre-filtration is the first part of the system. Its job is to remove the heaviest or largest solid wastes from the water column. Pre-filtration protects the pump and biofiltration from heavy solids that cause restrictions and clogging, lowering overall performance. Water can be removed from the pond through skimmers, bottom drains and, in some cases, mid-water drains. Each of these has varying flow rates depending on size and type.

Skimmers can be gravity-flowed to another pre-filter or direct suction to the pump and vary from 1,500 gallons per hour to 5,000 gph, with the higher flow rates being more hazardous to the fish. Skimmers vary in pre-filtration ability depending on their type. A skimmer’s primary job is to create surface tension, drawing water across the top of the pond to collect floating debris.

Some have just a basket or net to trap the larger debris while others have additional ability with the inclusion of pads, matting or other media types that can trap particles. Skimmers with pads, matting or media will do some bioconversion, but I generally do not count them in my calculations because they get serviced more regularly than other components in the design. The cage around a submersible pump and the leaf trap on the intake of an external pump are also primitive forms of pre-filtration.

Bottom drains should never be plumbed as direction suction to a pump. This leaves only the leaf trap on the pump as pre-filtration and a leaf trap does not
adequately have the capacity or the particle separation capability required to do the job. A leaf trap’s only job is to protect the pump’s impeller. There are several types of pre-filtration for bottom drain circuits, but the most common form of pre-filtration is gravity flow to a settlement tank with additional particle trapping or separation in the tank. Other types are rotary drum
filters, vortexes and floating micro-screens. There are also above-ground pre-filters that can be utilized in a rebuild or when gravity flow isn’t possible because of space constraints. Gravel beds are also a form of pre-filtration that also includes bioconversion. All pre-filters should have bottom drains or discharge pumps for easy cleaning.

An air-lift-operated current installation.
An air-lift-operated current installation.


Biofiltration is where the work gets done. A biofilter’s job is to provide a
protected home for the bacterial colonies, allowing them to expand to a size equivalent to the volume of ammonia produced in real time by the fish. They fall into two basic categories: static-trapping and aerated. Static-trapping filters have two jobs — bioconversion and water polishing or clarity — and they must be sized accordingly. There are many types of trapping filters, but all have a speed limit.

If water passes through them too quickly they will do the conversion but they won’t trap very well, leaving particles suspended in the pond, reducing clarity. For open up-flow filters, a good rule of thumb is a maximum flow rate of approximately 630 gph for each square foot of cross-sectional surface area. This, of course, varies based on filter media type and design, but it’s a
good place to start.

Aerated biofilters are filters that add oxygen or use air to circulate water within the filter. They come in several styles, from aerated static media to moving beds and shower filters. Aerated biofilters do a huge job of bioconversion because the bacterial colonies need oxygen, as do the
fish, but they won’t do any trapping. While extremely productive in bioconversion, they should not be used by themselves.

Aerated biofilters need good pre-filtration in terms of fines trapping because solids and fine particles just pass through them. They must be used in conjunction with a statictrapping filter for good water clarity. For instance, a moving bed can be used in series with an under-gravel system but cannot be used as a stand-alone biofilter for a system with a bottom drain and no pre-filtration. All biofilters should have bottom drains, and some should have upper rinse drains to adequately clean them.

Another air-lift-operated current jet; this one is installed and running.
Another air-lift-operated current jet; this one is installed and running.


Most ponds suffer from a low dissolved oxygen content. This is one of the most common problems I see when rebuilding ponds. Bottom drains can be aerated with an external pump, aerated biofilters add oxygen, and some current jets can help oxygenate the system. Aeration tubing can be installed in under-gravel systems and free-standing air domes and air rings can be placed on the pond floor.

In recent years I have built a number of ponds that run on air-lift pumping systems that both flow the water through filtration and oxygenate the water column with the same energy. Some have been split systems with part of the water flow created from air-lifts and the rest through conventional pond pumps to
power a waterfall filter.

Current jets or returns are an important consideration when designing a pond.
Areas of low circulation have poor water quality and create dead zones. Recently I rebuilt a pond and had no way of effectively running water returns to the corners. I built two small air-lifts and installed them into the pond corners. They pulled water from the bottom corner and sent it out over the surface for circulation and oxygen. I used the same 25-liter-per-minute air pump that was being used for the small air dome, so no energy increase was necessary
and the pond gets oxygenated all the time.

Incorporating these concepts into the design up front helps ensure a successful project long-term.

Kloubec Koi Farm

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