New pond builders rarely learn the significant differences in water gardens vs “ideal” koi ponds. We tend to borrow ideas from aquarium technology, water gardens, water treatment, industrial processes, and even the ideas tried and tested for a long period of time. But one of the most common mistakes we make due to space limitations or even just laziness is the circuit from bottom to returns. What happens outside the pond is just as important as the design of the pond basin.
Small ponds will usually have a housed submersible pump that feeds a series of filters. Another common design is to use a retro fit bottom drain connected to a powerful external pump which feeds a bead filter and/or DIY filter (aka Skippy filter or box filter). This is my particular design and I am constantly having to clean the combo mechanical/bio filter as well as remove the debris from the pump basket. I can tell you it is not the ideal situation for maintenance.
The ideal koi pond, even if only 1500 gallons, needs a properly situated bottom drain (Not a retro fit drain) flowing by gravity to a mechanical filter chamber or settlement chamber/vortex. Koi produce a lot of waste. If this waste can be moved with minimal agitation it is easier to remove from the water column. If we use a retro drain connected to a suction pump, the waste is chopped up and is harder to remove and often ends up in our bio filters making them less efficient. I will rectify this within the next couple of years when I expand the pond.
Another advantage of gravity fed systems is that they generally require slower water flow. This means less energy used by the pump, and a smaller UV unit required. You can increase/reduce the flow by using variable speed pumps or by using a ball valve after the pump to create pressure head resistance. The bottom drain pipe to the first chamber should be as large as possible, usually 4″ pvc, so that the solids moving through the system can flow freely. Overtime this diameter may be reduced by bio layers and algae adhering to the sides. You can put your additional mechanical filters and bio filters after the first chamber before the pump, or put them after the pump. If using chambers in series before the pump you should learn about the properties of draw down due to resistance between each chamber.
This is also a good time to review raceway filter designs. A raceway is a long but shallow horizontal flow filter that sits outside the pond but at same water level. Generally dirty water is gravity fed from a bottom drain into one end of the raceway which acts as the mechanical settlement chamber. Water then flows horizontally through various materials to remove debris and then through bio materials that complete the nitrification cycle. The pump is placed after the bio stage in raceway filters. Water after the pump can go to falls, returns/jets, or even additional filters like trickle tower filters. The entire filter can be dumped on a regular basis to keep it clean. Raceways are often covered by decking so they even offer a platform for viewing the pond.
As with anything related to Pond building please research all your options and think about the details outside the pond. Your fish will thank you for keeping their environment as clean as possible.
