Ponds by the Numbers: Rules for a lawless industry

Published on April 1, 2014


Given that we work in an industry that has no codes or specs and few guidelines, I think we should establish a list of arguable common rules that seem to generally work for those who follow them. Live water features cannot be designed and built as “cookie cutter” systems because of differences in geography, climate, location in a yard, landscape, shade, fish load, structural requirements and other factors — but we can start to talk about what generally works.

At the same time, we must consider that the hobby of koi-keeping has extremes. On one end of the scale, we have the backyard ponder with a water garden that sometimes has fish in it. On the other end, we have the koi keeper who owns expensive, show-quality fish. For our purposes here, we will only discuss ponds with fish because the requirements for water gardens without fish are much lower and can be in a category by themselves.

Fish ponds are first and foremost about the needs of the fish. Koi need lots of dissolved oxygen, good circulation and good filtration. Whether they are backyard koi or show koi, they have the same basic physical needs. Goldfish have basic physical needs too, but they seem to be able to handle worse conditions — mainly because of their smaller size. Goldfish are much more prolific breeders, though, and tend to get overpopulated at a much faster rate than koi. For this reason we will focus on the needs of koi in this list of rules.

3 Feet: This is my minimum depth for building a pond

 Birds of prey such as the Blue Heron have long legs for a reason. They can stand completely still in a body of water until the fish forget they are there and then pluck them out easily when they swim near. 
A deeper pond prevents that from happening.

 Sunlight affects 100 percent of the water down to about two feet. By building a pond deeper than two feet, you give the system a cushion of water volume unaffected by sunlight.

 Volume is your friend. Deeper water gives you more volume in the same footprint, and a larger volume of water stays more stable in both temperature and water quality over time. Stability is good!
 Shallower areas, including plant shelves or steps for safety, can be built and are usually limited to the edge or end(s) of a pond, depending on shape.

O Minutes: This is the amount of time a pond pump should be shut down or placed on a timer.

 The aerobic bacterial colonies that live in the biofilter — doing the work of converting the ammonia produced by the fish to nitrites and then into nitrates — need oxygen, food and minerals to survive, as do the fish. It is estimated that they can start to die off in as little as four hours. They will grow back, but putting a pond system through this constant death and rebirth cycle is hard on the system and leads to poor water quality.

 Even if the bacteria don’t start to die off, each time the system is shut down for a period of time it starts to go anaerobic. On startup, the instant surge of water through the filter(s) will dislodge debris and send it back into the pond, usually with an accompanying smell.

 Once started, the system should run forever. The pumps should only be shut down during certain phases of cleaning as necessary.

5 to 10%: This is the ratio of filtration to total pond volume.

 A pond with good quality filtration and maximum dissolved oxygen content can get away with filtration in the five to seven percent range. This is a case where more is better.

 Pressurized biofilters consume oxygen and are sealed, so when using them always allow for additional oxygen somewhere in the system. It’s my opinion that most pressurized filter manufacturers over-rate their equipment, so I cut their performance numbers by one third to one half when I have to use them.

## 4 to 6 feet: This is the average radius, around the drain, a four-inch gravity flow bottom drain flowing approximately 3,600 GPH can effectively service. ##

 A three-inch drain line is half of that and so on. I have built several very small ponds with two-inch gravity flow drains, but they don’t work well. These days, even on small ponds, I stop at three-inch drains. They are easier to maintain, and with the lower flow rates of small ponds, they need more maintenance and cleaning — which is much easier with a three-inch drain than a two-inch.

 On ponds that are direct suction straight to a pump, you just go by the flow rate. I don’t recommend direct suction without a pre-filter other than the pump’s leaf trap, because it is dangerous to the fish and requires much more pump maintenance.

3,600 GPH: This is the volume I give to an average four-inch gravity flow line

 A four-inch gravity flow bottom drain, skimmer or mid-water line will flow approximately 3,600 GPH of water when the water level in the tank it is flowing to is approximately one inch lower than the surface of the pond. Water seeks its own level. As a pump removes water from a pre-filter tank or other filter, the water in the tank will drop and the water in the pond or previous tank will try to equalize back to the original level. When it reaches approximately one inch lower than its static starting level, the water flowing into the tank from the pond to replace it will reach this rate excluding any pipe friction head. The shorter the pipe the better. A longer pipe will necessarily flow a little slower because of increased pipe friction.

 Consequently, a three-inch pipe with half the cross-sectional area will flow approximately 1,800 GPH. A two-inch pipe will flow approximately 900 GPH and a 1.5-inch pipe will flow approximately 450 gph.

1,500 GPH: The minimum flow rate required to get most skimmers on the market to work properly.

 A skimmer’s job is to create surface tension and pull floating debris toward it from across the pond. The surface water of a well-skimmed pond will move at a faster rate than the circulation below it.

## 200 gallons: The minimum volume of water an average adult koi needs to adequately survive over its lifetime. ##

Anything below this is the number at which a pond begins to be overstocked.

 Overstocking leads to unhealthy and diseased fish.

 The larger the fish, the more volume it needs. Many serious show-quality koi owners typically run 1,000 to 2,000 gallons per adult fish.

## 1,000 GPH per linear feet: This is the minimum number of GPH required to make a waterfall or spill look good. ##

A one-foot-wide waterfall needs a flow of at least 1,000 GPH, while a two-foot-wide waterfall needs at least 2,000 GPH, and so on.

## 2,100 GPH per amp: This is the rate at which the most efficient pumps on the market move water at a head of approximately four feet. ##

 Many manufacturers list various head heights for their pumps, which makes it confusing to compare. It would be much easier if all manufacturers rated their pumps on a standard of three or four feet of head, since that is the typical range of waterfall height.

 Waterfall or spill height above pond level (“static head”) alone is not the deciding factor in measuring total head. Pipe friction or “dynamic head” is a part of the equation, but it’s much more difficult to calculate so I don’t. I typically build with proper pipe diameters and use pipe runs as short as possible, then add 10 to 15 percent to my static head.

 This also does not include the use of pressurized filtration systems. Pressurized systems require a lot more head, or “pressure.” The manufacturers of pressurized equipment are lacking in willingness to issue flow charts similar to the ones used by pump manufacturers to rate their equipment. Pressurized filters should have ratings that express the amount of head in relation to the flow through the filter.

1 to 2 times per hour: The average turnover rate of total pond water

 The turnover rate is defined as the number of times the total volume of pond water flows through filtration in one hour.

 This generally does not include water that is bypassed directly back to the pond past the filtration system. This is a little bit of a gray area, though, because some bypass water can be used for very useful purposes: current flow, for instance, or aeration not involved in filtration, such as a waterfall or feature without a filter.

 Smaller ponds need a higher turnover rate than larger ponds. Things happen faster in a small pond, and smaller ponds are usually more likely to be overstocked. Small ponds are also less stable, as their temperature fluctuates more. It’s more difficult to make both a skimmer and a bottom drain work well on a flow of less than about 2,400 gallons per hour (or GPH). For me, this means that a 1,000-gallon pond will have a turnover rate of about 2.4 times per hour.

 Typically my larger ponds run approximately 1.5 times per hour, all through filtration.

 Extremely large ponds and small lakes can run much lower rates and the rules apply much differently.
 The higher the dissolved oxygen content, the lower the turnover rate can be. Oxygen is your friend.

630 GPH: This is the maximum rate of flow for one square foot of cross-sectional surface area for a static trapping biofilter.

 Biofilters are in two basic categories: Static Trapping and Aerated.
Static trapping biofilters house your bacterial colonies but also trap fine solids for water clarity. Aerated biofilters don’t trap any solids but have a higher rate of ammonia conversion. Consequently, aerated biofilters have no speed limit as opposed to static trapping filters.

 Running static trapping filters at a higher rate prevents them from doing the best job they can at trapping. When trapping fine particles, slower is better.

 The number of biofilters needed for a specific pond is directly related to the volume and turnover rate. I try to use both types whenever possible.

1,000 GPH per linear feet: This is the minimum number of GPH required to make a waterfall or spill look good.

 A one-foot-wide waterfall needs a flow of at least 1,000 GPH, while a two-foot-wide waterfall needs at least 2,000 GPH, and so on.

## 7,000 GPH per amp: This is the approximate amount of flow per amp through filtration with a properly designed system run with air-lifts. ##
 These systems are relatively new to the pond world, but I regularly build and use them. They have their pros and cons, though.
 An air-lift system cannot run a waterfall or any equipment that requires much head or suction.

3 thoughts on “Ponds by the Numbers: Rules for a lawless industry”

  1. Hello,

    Just came across your article. Great information. I had a question – I’m about to expand my pond from about 1000 gal to about 2000. I have an Ultima 4000 gallon filter which I will set up once I expand my pond. I have a kidney bean shaped pond that is 11ft x 6ft(avg width) x 2(avg depth). I am going to make half my pond 3ft at the shallowest and the other half sloping down to 3.5 or 4ft at the deepest. This would give it about 1800 or so gallons.

    My question is – Do you think turning over my pond volume 2 times in an hour is sufficient? I want to grow my kois big(only have 4 in there right now that are about 18 inches but am thinking of adding 4 more once I expand) and at the same time, maintain excellent water quality – which I currently can’t seem to get the ammonia down to a 0 very often. Most of the time, it would go down only to .25, mainly because my current Ultima filter is for 1000 gallons. I do have a couple of air stones, and a submersible pump rated at 2000gph. I also have a nice waterfall. My new pump will be external Performance Pro, rated at 3780gph – 3840gph @ 15ft head(taking filter head loss into account as well). Thank you in advance for answering my question and any other suggestions would be appreciated. – Sib

    1. Avatar photo
      Lora Lee Gelles

      From Kent Wallace:
      If your ammonia level won’t go down you don’t have enough bio-filter surface area. I would run both filters not just the new one. Split the water coming out of the pump between both filters slowing the volume down through each for better fines filtering as well as increasing the bio conversion with two instead of one.

  2. I found this article when I first researched a pond about 5 years ago. Later I could not locate it again. but remembered most of it. 4 years later I have upgraded 2 times, enlarged 2 times water fall was 2 foot its 6 foot now. This morning I found your article again and everything has held true awesome article – thanks. The information is priceless.

    FYI in 20 years this is the first time I have ever responded or gratified any article about ANYTHING,

    The only thing you don’t touch on is UV clarifiers / sterilizers and so much controversy about before and after filter bio fall filter. GPH is real confusing when reading manufacturers labels. It becomes a sterilizer with less then optimum GPH.

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