Koi for Ponds can be formal shapes like long boxes and round bowls or informal Japanese style garden ponds as long as there are no dead water corners or stagnant locations.
However, ponds imitating bent rivers or L shape are best for koi because the koi can’t see around the corner and will spend all of their time swimming back and forth from one end of the pond to the other looking for food. This exercise is good for koi. The old style koi pond with in-pond downflow, gravel filter at one end of the pond separated by a block wall that stops 6-12 inches below the surface accomplishes this same curiosity in the koi who can’t see thru the block wall so are constantly swimming into the filter area. Koi in a round or kidney shaped pond will do Ok but jets may be needed to make the koi swim more against a current for better growth and health. Any shape pond can work but there are some rules.
Make the walls vertical or almost vertical. If you add a ledge 18 inches deep to place water plants into your koi pond you are providing access to raccoons and blue herons to your pond and koi. If you must add plants, use the floating plastic baskets designed for water lilies and do not build a ledge into your pond if you can avoid it. If you must have a ledge, there are electric fence devices and motion detectors with either sound or spraying water to combat these predators to some degree. But even with them, you may lose some koi to predators. Local building codes usually treat koi ponds like swimming pools if they are over 3 feet deep.
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So the rules for concrete swimming pools including permits, rebar, bond beam perimeter and grounding of rebar will apply. You should also ground your pump. You MUST use GFI or Ground Fault Interrupter plugs or circuit breakers for any electrical used for a pond for not only the safety of the koi but for yourself. Many of the materials, processes and codes applying to swimming pools will also apply to koi ponds but DO NOT let a swimming pool contractor design your pond and filter system. Use someone who knows pond design or learn and design it yourself and then use swimming pool contractors to build to your design. Koi pond and swimming pool systems have much in common but the differences are very important to proper functioning and maintenance of a koi pond.
One difference is a koi pond requires a bowl shaped bottom sloping to a bottom drain with no corners or flat areas for proper automatic bottom cleaning. Swimming pools have flat bottoms that would require constant cleaning for a koi pond owner. Swimming pools use expensive to operate, high Hp pumps and pressure filters intended to mechanically filter the water for 4-5 hours each day. Koi ponds use inexpensive to operate, low Hp pumps feeding filters that grow beneficial bacteria to treat the water 24 hrs/day. Always design 3-4 inch bottom drains into your pond at the far end from the water source or waterfall and every 20 feet across the bottom with the bottom of the pond sloping to these drains. A surface skimmer will also be required to keep the surface free from the dust and debris on the surface of your pond.
Place the skimmer furthest from the waterfall or where the wind usually blows across the pond for maximum efficiency. Either a swimming pool skimmer or a no niche skimmer may be used. The no niche skimmer requires pump suction to operate properly. A swimming pool type skimmer line is often Y branched into a line from a bottom drain so the flow from the bottom drain pulls the flow from the skimmer. Rocks, bricks or material surrounding the pond must be raised to not allow any runoff form the surrounding area. A 10inch drop from the top of the rocks surrounding the pond to the surface of the water will resolve any raccoon or cat problems and will usually keep the koi from jumping out.
Koi Pond Materials
Prefabricated fiberglass ponds are available up to 2000 gallons and can be installed above or below ground. PVC or Butyl rubber liners are also available to create in ground ponds and will last 20 or more years. Bottom drains are available for liner ponds. Many ponds are made with concrete blocks placed on a concrete base. However most ponds are concrete, shot-crete or gunnite. Concrete ponds with Red Label additive will harden quickly and tight so no sealer is required and they can be filed immediately. Shot-crete or gunnite ponds are porous and will require a sealer on the inside of plaster, Thoroseal (plastic cement compound), Hecht rubber or other sealer compounds for water basins. Hecht rubber requires proper surface preparation and primer for proper adhesion.
All concrete, shot-crete and plaster ponds will require Muriatic acid wash to remove lime and lye from the surface layers or gallons of vinegar in the first water fill to do this task over 3-4 weeks. This acid wash is done prior to coating the pond surface. People have also laid their own fiberglass pond using resin and matting because they could do it in stages. Plaster must be applied in one application to provide a good seal and must not ever be exposed above the water line so the top 8-10 inches of your
pond will probably require Thoroseal which is not sensitive to cracking with air exposure like plaster. This is the reason swimming pools have tile around the edge. All of these products work if instructions are followed.
Caution – do not ever allow treated wood to come in contact with pond water, it contains arsenic. Bare redwood or cedar is OK if not treated. Know what is in your stain or surface coating on decks that could have runoff into your pond. Even some decorative bricks have been known to poison ponds when in constant contact with pond water. Do not use copper or galvanized pipe for koi pond plumbing, use PVC or ABS plastic pipe. Some brass valves are probably OK.
Some water garden designers suggest rocks and gravel in the bottom of the pond for its natural
look. Rocks are bad because your koi will scrape themselves on sharp edges. In nature, streams have quickly running water and a mud bottom to clean the debris from this quagmire. Your concrete or liner pond has neither fast flow over or thru this material so it will just collect debris and become a smelly cesspool over time which will require periodic emptying of the pond and cleaning of this material while your filter bacteria die due to the pond being down. This cesspool will also grow hydrogen sulfide gas that will kill fish and anaerobic bacteria will grow, which is not good. So I cannot recommend this type of pond.