Building a concrete block filter unit.


A Guide to Modern Koi Pond Construction Building a concrete block filter unit
This page shows the building of a concrete filter unit, and the method is the same even if they are in a straight line.
	A 6" thick reinforced concrete base has to be installed in the same way as was shown for the glassfibre filter system. The only difference this time is that we have to include 2" diameter pipe work for the bottom drains. It can be seen in this case they have been installed and carefully covered with cloth so that no debris can enter while other work is being progressed. The pipes have been left too long, but they will be cut off when the rendering is completed, and before the fibreglass laminate work is commenced.
	The blockwork for the filter walls has been started using 18" x 9" x 4" concrete blocks laid in the normal manner so that the walls are 4" thick. In some cases members prefer to use the blocks in the same way as for the pond walls so that the blocks are 9" thick for the outside walls, but this is not generally considered necessary. The vortex chamber is already in position, and as the overall height of this is higher than the filters then the base has already been put in, and is lower than the filter base. As the pond walls are three feet high then the base of the filter is level with the ground level.
	In this photo you can just see the three filter bays. The first one is on the left of the photo, and the other two are on the right. Many filters are built in a straight line along the side of the pond, but in this case because of the triangular shape of the garden it was necessary to create it in the shape shown. In this case the vortex chamber has a cover, but this is unusual as generally they do not have one supplied with the unit.
	The photo on the right shows the filter being rendered. Most members find that they can do the blockwork themselves, but there are very few that feel confidant enough to do the rendering themselves. For that reason most koi-keepers employ a professional to do this for them.
	In this photo the rendering has been completed. It should be noted that the division walls which will form the weir has been nicely rounded, and this shows up even clearer in the bottom photograph. The main reason for this is that the fibreglass does not adhere very well to right angled external corners.
	This photo shows a fibreglass division plate that has been made up some time before and is ready for putting in position when the walls are fibreglassed.
	The fibreglass has now been completed, and the filter unit is complete. The division is in place alongside the weir. The water will flow over the weir and under the division plate.
In this example the filter unit has been fibreglassed, but if you prefer then after rendering then the surface can be treated with G4, or any of the special paints available for this. A fibreglass division plate has been used, and it is worth considering even if you are not going to fibreglass the walls of the filter. The division plate can be cemented in when doing the rendering. The other alternative is to use two 4 inch (110mm) pipes to flow the water from the top of one bay to the bottom of the next. This should be satisfactory for flow rates up to 4,000 gallons per hour.
Let us us now have a look at the Nexus units.
This page updated January 2004
This website started with a booklet which I made up originally for the help of those members of the BKKS - South Wales Section who were intending to start building a koi pond, and as the website has expanded more information and articles from other people have been included. This is for general guidance only, and we do not take any responsibility for problems that might occur by following these ideas. It is important that you check out everything in your area, and check on local laws and rules that may apply.
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