Concrete needs a little special care when used to make fish ponds, due to the possibility of lime leaching into your pond water. Concrete is easy to shape and maintain but requires reinforcement on the sides of the pond while it is drying. Your concrete fish pond walls must be thick enough to prevent cracking during repeated spring and autumn thaws. Make your concrete pond deep enough to prevent herons and other birds from using it as their private sushi bar.
Photograph your pond site from the house, the street, the rear end of the garden and from any window that overlooks it. Use the photos to decide where the pond will have the most visual impact. Check placement of any existing trees or shrubs that might blow debris into your pond. Locate your pond away from pine trees, as the needles will pass through your filter and get into your pump housing.
Contact all local utility companies and ask them to mark locations of any underground pipes running through your property. Follow UK Health and Safety Executive excavation regulations, including proper shoring of the trench or excavation to prevent injuries and deaths from collapsed walls.
Dig deep if you are going to stock expensive ornamental fish such as koi. Heron think garden ponds are free sushi bars, and will quickly decimate or even eliminate all your fish in just a few hours. Make most of your pond around 1.8 metres (6 feet) deep with straight sides and no ledges. Add an exit ladder instead of ledges to make it easier to get out of your pond when you need to clean or repair it.
Use a rounded-bottom design that is sloped toward the middle, one side, or one end of the pond and have at least one of your pond drains installed at the deepest point. This will save you hundreds of maintenance hours, as debris will settle in that spot instead of all over the bottom.
Lay out your plumbing, air lines, pump housing and filter design before you pour the concrete. Try several configurations before you install everything permanently. Make sure everything can be easily reached for repairs and maintenance. Once you are sure everything is where you want it to be, hook it all up, put the pump in a bin of water as close to its proper position as possible, put the drain line in another bin and attach a garden hose with the flow set to match your expected pump and filter flow. Check for leaks and other design flaws. Correct any problems before you pour the concrete.
Position overflow basins at the low end of the slope of the area where you intend to install your pond. Consider whether to create a dry stream bed overflow, which will accent your pond whether it is full or not. Dry bed overflows create habitat for toads and other beneficial garden creatures, and prevent storm water surges from flowing back towards your home, patio or garage.
Position any biological filter, refugium or spawning tanks together at one end of the pond, and conceal them with piled stone or some other attractive structure of your choice. A refugium is an add-on tank that shares the same water as the main tank, but keeps more delicate or spawning fish separate from more aggressive species.
Install ground fault circuit interrupter (GFCI) outlets and run any wiring needed from the house to the pond site while you still have the backhoe and trencher on site.
Line your pond with an impermeable geomembrane that is compatible with concrete. Pour a layer of sand on the bottom of your pond and tamp it down well using a plate compactor. Repeat this for a layer of pea gravel. Lay your reinforcement material in the bottom, and attach it to the sides of the pond. This may consist of rebar, layered chicken wire, welded wire mesh or welded wire fabric.
Have a building contractor pour your concrete. If possible, have all concrete poured in one day. If not, pour the bottom first, then the side walls, and make a thick concrete seal between the two pours to prevent cracks and leaks. Pour at least 15 cm (6 inches) thick for the bottom.
Allow the concrete to dry for three to five days or more, depending on weather and the recommendation of your contractor. Apply a sealant to prevent alkali leaching.
Fill your pond with water and run the pump and filter system for three to five days to allow the water to condition, and to absorb sunlight to moderate the water temperature. Test your water quality and ensure that it matches both the temperature and pH of the water where you will be obtaining your fish.
Place edging rocks and any overhanging or border plants around the pond to conceal any area that detracts from the overall beauty of your pond. Add one or two inexpensive test fish to your pond and watch to see how they react. If they seem OK, proceed to stock your pond.
Stock fewer fish than your pond can support. This allows for spawning, water quality problems, and fish growth. If your pond ever becomes overstocked due to fish fertility or fish girth and length, build a second pond and move the excess fish to it. Fish are overstocked when they cannot swim freely without agitating one another, when they begin to show aggression, and when larger fish crowd smaller fish away from food and shade sources.
Concrete must be cured for up to 60 days before being used to house aquatic organisms or the water will be too alkaline. Concrete ponds in clay soils must have a sand liner between the soil and the concrete.