Pond Design Information for a Two-pond Filtration System

Published on July 21, 2009


Filters that are simple and easy to maintain are the best design no matter what kind of filter you are talking about. Whether it is a biological, mechanical or even a plant filter – it makes little difference when it comes to designing one – remember that the ultimate goal is simple and easy maintenance. Here I want to talk about all three filters wrapped up into one design that can also be a beautiful addition to the landscape.

The basic design incorporates the main koi pond coupled with another pond that serves as a mechanical filter by settling out the solid particles. I will term this pond a “settling pond.” Within this pond lots of plants will be added. These plants and their associated roots will also serve as both the biological and phyto filter. The two-pond concept is not new, but is often overlooked as a filtration solution. The following discussion on how they are set-up and operated should help you in the design of future koi pond systems.

The koi pond can be almost any type of a simple and open design that incorporates a high flow rate through the pond of at least one pond volume per hour. This kind of flow assures that adequate circulation occurs in all parts of the koi pond. This maintains high levels of oxygen in the entire water column and it also serves as a sediment and waste “mover.”

Another aspect of the koi pond design should incorporate a depth of at least 4´. By having an adequate depth the koi develop normal swimming muscles by traversing the vertical layers of the pond, as well as the horizontal elements. Also, in this koi pond, at least one bottom drain could be installed. Drains are installed for primarily two reasons – as an intake for the water pump as well as a method of removing bottom sediment. This drain is not absolutely necessary for sediment removal due to the koi’s habit of keeping particles in suspension by their feeding behavior. Koi love to pick up bottom sediments looking for food and then spitting the non-food sediment back out thereby constantly stirring things up. These sediments are almost always in suspension inside the koi pond.

Now for the interesting part on how we remove the sediments. Instead of installing high maintenance mechanical and biological filters, I like to build the additional settling pond as the upper pond. It will flow into the main koi pond. This pool can be built as large as you want, but there is a limit on how small it should be. A pool of less than 1000-gal would be approaching the lower size limit of the settling pond.


The important principle that I want to convey is that you want to be focused on settling the sediment out in this pond. Let us use, for example, a 5000-gal koi pond and a 1000 gal-settling pond. We would have a minimum pump output of 6000 GPH (remember – one pond volume/hour) from the koi pond. This flow would need to be greatly reduced through the settling pond for the solids to settle out. The rule of thumb is to set the flow so that its volume flows through the settling pond every 5 to 10 hours. Flows much higher than this will cause the water that contains the sediment to simply flow through the settling basin with very little sediment dropping out of suspension.

In order to accomplish the reduction in flow, most of the output in my example of a 6000 GPH water pump would need to be diverted to go around the 1000-gal settling basin. On the output piping from the koi pond, a Y-fitting is installed and a small part of the flow is allowed to go through the settling basin. The rest of the flow bypasses the settling pond and is injected into the stream that flows directly back into the koi pond. Of course, this is regulated with a ball valve and can be adjusted to differing conditions.

In the 1000-gal settling basin, the flow coming in would need to be reduced to less than 200 GPH or slower. That means the suspended sediment in the water that is diverted around the settling pond will make another trip around the system. It will eventually end up in the settling basin because the koi keep it stirred up in the koi pond. The settling basin can remove almost 100% of the sediment in the water that goes through it. Within a day or so, all the water in the system will go through the settling pond. Patience is a virtue with this slow rate of flow. However, it will all eventually end up in the settling basin.

Design of the settling basin is crucial to the efficient removal of the sediment. A sediment pit must be built into the settling pond for efficient removal of the muck. This pit would be located in the slowest water movement area of the pond. It could be in the center or at the far end from where the water is introduced into the pond. This pit needs to be 6? to 12? deeper than the rest of the pond and be from 1´ to 4´ square depending on the size of the settling pond. The best design uses a bottom drain located in the bottom of this pit. If a drain is not installed, then a wet/dry vacuum could be used to remove sediment. However, I prefer the bottom drain for easier removal.

It is very important to gently slope the bottom of the settling pond to the sediment pit. This slope should be approximately a 2/12 pitch. If it is much greater than this it is hard to walk in the pond when necessary.

As stated earlier, the larger the settling pond, the faster we can pump water through it. These flow rates can be adjusted with the installed ball valve depending on how efficiently the sediment is dropping out

Plants in this settling basin are another very important element in the design. Plants like water lilies, water hyacinths, water lettuce, submerged plants and The Grid filters (uses hardy plants with exposed roots, see POND Trade magazine Jan/Feb 09 issue) are excellent phyto filters. The plants help slow the water flow even more, so the sediment has more opportunity to drop out. By adding plants you have also created a beautiful water garden in addition to a koi pond. Plants provide a huge area for biological filtration to take place, especially plants where the roots are exposed. Placing plants directly in a koi pond is a gamble as to whether the koi will eat them while plants added to a settling pond will thrive.

Only small fish should be introduced to this settling pond. I have used Calico Shubunkins with high success without them keeping the sediment stirred. I do like to flush the sediment pit very frequently so that the fish do not have as much chance to disturb the sediment. How often you flush with the bottom drain, or the use of a wet/dry vacuum, will be a judgment call depending on sediment buildup. The flushing should continue until the water runs clear, which will only take a minute or two. Not much water is lost in this process.

A two-pond system design could present a problem with the lower koi pond overflowing when power is interrupted. How much water is lost will depend on the stream depth at the mouth of the stream between the ponds. The upper settling pond will continue to flow down the stream until the level equalizes to the depth at the mouth of the stream. If this depth is kept to a minimum then the amount of water lost is not a factor.

It is relatively easy to retrofit an existing koi pond with a settling pond. This settling pond does not have to be immediately adjacent to the koi pond but you do have to link the two ponds together with either a stream or piping. The idea is appealing because the added settling pond also serves as a beautiful addition to any landscape.

This two-pond system provides all the filtration that is necessary for a koi pond. With plants, which are a must for this to be successful, you have all three filters – phyto, mechanical, and biological. Maintenance is relatively easy. Periodically flushing the sediment pit by opening a bottom drain is a snap and the trimming of the plants is a very enjoyable experience. These are the only two maintenance items that will need regular attention. The idea is relatively easy to sell due to the beauty and ease of maintenance.

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