Water Lily

Koi Pond Filtration

Most experts, when talking about filtration for a koi pond, will advise using add on mechanical and biological filtration. The biological filter is usually a large drum filled with bio media and a spillway for a waterfall. Mechanical filtration is done with a skimmer and or pressure filters. Another popular product is a UV sterilizer sworn to keep the water crystal clear. All of these products have a purpose that will be described further, but I would like to focus on the natural biological filter. The best thing about using natural media for biological and mechanical filtration, yes I did say "mechanical filtration", is the more natural and appealing look. It also gives the Japanese koi an environment where they can forage constantly.

Gravel For The Bottom

Putting gravel on the bottom of a koi pond serves two purposes. The first benefit from a bed of gravel is that it becomes a biological and mechanical filter. Just as in a fish aquarium the gravel becomes a natural media for denitrifying biological activity, only on a much larger scale. It also traps micro particles which settle to the bottom of the gravel bed. There are many people that argue against using gravel on the bottom, saying that it will cause a sludge build up. They also advise using a bottom drain for the same reason. It is the muck that is under the surface of the gravel that hosts organisms that the koi will eat. It also hosts anaerobic denitrifying bacteria. Top Japanese koi breeders use mud ponds in fertile valleys to raise their Niigata koi. They don't have bottom drains either. In fact they use a bentonite clay to replace depleted minerals regularly.

The other major benefit of putting gravel on the bottom is hiding the liner giving it a more natural look. Adding larger rocks to line the sides also adds more nooks and crannies for biological activity while hiding the liner further. Now with the liner completely hidden it is starting to look more natural. Most people worry about the rocks poking a hole in the liner, and how would you ever find it without dismantling the whole pond? Using a good 45 mil pond liner and a good liner underlay will prevent any leaks.

Most leaks occur at the waterfall or running over the sides in places that settled or plants that became thick enough to back the water up. I use plain old river rock in various sizes from medium to large. Growing up in the Ozarks I was used to gravel bottom creeks and sand stone which come in several colors from white to pink to dark brown. I would encourage the use of rocks that are indigenous to your area. It makes it look less out of place. When placing large rocks in the pond cut a piece of liner about the size of the rock to give it an extra layer of protection.

Watercress in Mid April Watercress

Plants For Filtration

Plants do a great job of cleaning up fish waste and whatever else ends up in the pond to decay. They also add to the natural beauty of the pond. Water lilies taken out of the pot, rinsing the roots, and placing directly in the gravel bed will use the nutrients from the fish waste and dead plant material. They also give the koi some shade and a place to hide when startled. The roots will spread under the gravel larger than the canopy diameter. The water lilies will bloom more without having to fertilize. The only time to fertilize a lily is when you first plant it. I have never had koi eat any of my pond lilies. Water hyacinths have an extensive roots structure for pulling nutrients out of the water. I usually plant these in the stream part of the pond. This may seem strange to put a floating plant in a shallow high water flow area. Koi tend to eat the roots of this plant and that is the reason for putting them where the koi will not go. They tend to bloom less and grow taller when they are not floating. Hyacinths bury their roots deep into the gravel and do an efficient job of cleaning the water.

Watercress in the Fall Watercress

Watercress is probably one of the best plants for the koi pond. Japanese koi love to eat this plant which will grow faster than they can possibly eat it if planted correctly. This plant should be planted in the shallow area just before the deeper part in the gravel to one side. The plant is slow to start in the spring and could be eaten before it has a chance to flourish. Once the water temperature comes up in mid to late spring it quickly takes off. This is about the time that the appetite of the koi increases dramatically. Watercress will still have to be thinned at times. This plant has a root structure like a thick mat that will back up the water causing it to run over the side. Thinning the plants periodically removes toxins from the water that in an aquarium would be done with activated charcoal. Marginal plants are more for decoration and help to hide liner to make the pond look more natural. They do play a role in water filtration. I try to use hardy plants that don't have to be replanted with the exception of the water hyacinth.

Mechanical filtration is best handled by using a skimmer filter. Skimmers filter leaves, windblown debris and proteins off the top of the water before they have a chance to break down. Ponds with a heavy fish load may require a biological filter built into the waterfall or an external filter. The Ultima II series filters by Aqua Ultraviolet, provide excellent biological filtration with a backwash and rinse feature.