Koi Pond – Concrete block filter units

There is a toss up between purchasing a fibreglass filter units that come pre-built but are expensive, or to build your own concrete block filter unit. Get as many quote as you can from dealers and suppliers, then weigh out whether you want to buy a fibreglass unit or to build your own.

Sometimes it is better to build your own as you can then make the filter system fit into the space available in your garden.

Building a concrete block filter unit

1. Lay a 6 inch thick reinforced concrete base as you would like the base of the pond and for the glassfibre filter system.

2. You will need to include 2 inch diameter pipe work for the bottom drains.

3. Begin building up the filter walls with 18″ x 9″ x 4″ concrete blocks with about 4 inch thickness.

4. Position the vortex chamber. Remember that the concrete base of the filter must be level with the ground.

5. Build up the walls of the filter, allowing overflow from between the chambers.

6. Render the filters as you would like the pond. Hire a professional if you are not familiar with rendering the filters.

              Mix three parts sand to one part cement for the render coat.

              Apply first coat directly onto the block work. Complete the surface, then rough it up with a trowel to prepare the surface so that the second coat will adhere to the first.

                  The second coat is made up to the same mix as well as a waterproofing agent and Fibromix. Follow the instructions as per required by the manufacturer. Fibromix consists of glass fibres (so be careful when using this). It should be broken up and added to the mix. The Fibromix serves to give stronger surface, and minimises surface cracking.

                  Apply this second coating mixture on top of the first. Allow to dry.

                  Smooth the surface with a blow lamp with a yellow flame (or a cigarette lighter) by burning any fibres that stick out, then rub the surface smooth. You must ensure that you have a smooth finish.

                  Use a pond sealer after to finish the surface. Follow the pond sealer manufacturer’s instructions carefully.

                        Method of application
                        Apply the first coat of G4 with a brush, and allow the layer to dry to a finger-tacky point or dry enough to walk upon carefully. This would be approximately 45-60 minutes.

                        Do the same with the second and third coating. Do not allow more thatn 4 hours between coats. G4 comes in Standard (brown translucent) and Mid Green – Black colour. So it you decide to use colour, apply 2 coats of standard and 2 coats of colour.

                        Allow to cure for 72 hours. After that, wash down the pond with plenty of water, drained, and then be fill.

                        7. Round the corner if you choose to fiberglass the chambers, as you would the pond as well because the fiberglass will not adhere well to sharp corners.

                                Applying fibreglass

                                  Cut the fiberglass matting to length first, allowing for a one inch overlap on each mat.

                                  Mix the resin and the hardener with a ratio that depends on the ambient temperature where you live. Because the resin hardens after mixing, do not mix too much. Mix just enough for about 30 minutes work, before you proceed with another batch.

                                  Apply to the concrete wall with a lambswool roller and cover the entire space where the fibreglass matting will be hung. Make sure that there is a one inch overlap for the subsequent matting.

                                  Work your way around the pond, applying matting to walls and floor so that you don’t have to stand on drying resin.

                                  Once the whole of the pond is fibreglassed, leave it until the application becomes tacky before applying the flow coat. The flow coat is a gel which has had wax added at the time of manufacture and leaves a smooth glossy finish.

                                  Apply the flow coat with a lambswool roller. Leave to cure.

                                  Check the flow coat layer after 1 – 2 days and sand any protruding splinters smooth.

                                  Leave for another day.

                                  After a day fill the pond with water and check for leaks.

                                  Repeat two to three times.