Q. I plan to build a pond using concrete blocks that will be 8 feet in diameter and about 4 feet deep, with a 4-foot high waterfall at one end. I hope to keep about a dozen koi. Filtration would be a sand filter that holds about 200 pounds of #20 silica sand and runs on a 1-horsepower pump.
Now for the questions. First, is the sand filter adequate to keep the water clean and healthy? Is the silica sand the right grade? Second, will I need to build more filters, such as a prefilter, a biological filte, or a plant filter? Third, is my pump up to job of circulating water? Lastly, what is the most effective way to lay out the plumbing for the pond?
A. Your letter did not make clear the precise shape of your pond. If you are designing a circular pond I estimate it will contain slightly under 200 cubic feet of water, assuming it is filled almost to the top. A square pond would contain about 250 cubic feet of water. Since there are 7.5 gallons of water per cubic foot, this puts your pond in the 1500- to 1900-gallon range.
Depending on shape, the surface area is between 50 and 64 square feet. A good rule of thumb for pond stocking is one 12-inch koi per 10 square feet of pond surface, assuming the pond has good biological filtration.
This suggests five or six average-size koi at most for your pond. Without biological filtration I would put only a single fish in the pond. Of course, you could squeeze a larger quantity of smaller koi into the pond, but within a year or two you will face the choice of trying to maintain a dangerously overcrowded pond or removing some fish.
Young koi raised in heavily stocked ponds do not grow optimally. Interestingly, many of the most attractive animals generally fail to compete successfully with their less attractive — but more aggressive — pondmates. Raising good quality fish can only be done in a properly stocked pond, not an overcrowded one.
The sand filter you plan to purchase is designed for mechanical filtration of swimming pools. The large number of tiny spaces between the grains of #20 silica sand will effectively trap particulates. However, sand filters work best with heavily chlorinated water because this reduces the overall particulate load by inhibiting bacterial and algal growth — a major source of particulate matter in aquatic environments. Under these conditions, a sand filter can be run for some time without backflushing.
Because chlorinated water will kill your fish and thus can’t be used, you will find yourself backflushing the sand filter twice a day, every day, after just a few weeks of initial operation. This is a waste of time and an unacceptable waste of water.
Some pondkeepers have tried replacing the sand with 1/2-inch gravel or lava rock. Although this alternate setup does cut down on the rate of particulate trapping and thus clogging, it also proves to be fairly ineffective.
As an alternative, some folks have had good success with paper cartridge pool filters for mechanical filtration. Backflushing, however, is still a major chore every few days.
If you are contemplating a garden pond in which plants, not fish, are the predominate feature, then no other filter is really necessary. A pair of small koi or several goldfish will do well in a pond heavily stocked with plants such as iris, water hyacinth, cattails or arrowhead. In this setup you can get some “in-pond” mechanical filtering by designing the waterfall to spill into the deepest part of the pond. Water going to the pump should do so at the far end of the pond, which should be about a foot shallower than the waterfall end. The water flow will be moving “up hill” to exit the pond, which will help heavy particulates drop out of suspension and give the water a cleaner look.
You can improve mechanical trapping by placing a detention basin between the pond and the pump. A 30-gallon garbage pail with some screening or foam padding inside can do coarse “pre-filtering” of leaves and the like.
If, instead, you have your heart set on a half-dozen koi in a fish pond, then a separate biological filter is indeed required. There are so many variations on design — including vegetative filters — that I cannot possibly provide details. You might check in previous issues of AFI for articles and columns I’ve written on this topic.
With very light bioloads, lava rock in your sand filter can prove sufficient for complete nitrification. However, the flow rate is so fast through the relatively small volume of the filter that it is a very inefficient design. And I doubt this setup would prove adequate for long. A more elaborate biological filter design would be in order.
I cannot estimate the flow rate for your pump. A rating of 1 horsepower doesn’t really mean much by itself. I would need to know some characteristic flow rates at several different head values.
Nonetheless, typical 1-horsepower sand filter pumps have very high flow rates at high head levels, so it is safe to say that the pump is significantly overpowered for your less than 2000-gallon pond. And because you will be running the filter 24 hours per day, that pump will add between 30 and 60 dollars to your monthly electric bill.
I would look for a pump that delivers about 1000 gallons per hour at your head rating, which might be between 5 and 10 feet depending on your setup. At most this would be a 1/2-horsepower pool pump, although there are many smaller pumps that would fit this bill.
Finally, I suggest you use 1½-inch flexible PVC pipe to run your pond plumbing. This can be buried along the outside edge of the pond. Some people bury it under the pond, but this means you have to tear up the pond to tinker with the plumbing.