More Than Just a Pond

By Kent Wallace

more than just a koi pond

According to anyone who has a fish pond — or calls me to come over and assess his pond and its issues — all ponds with fish are “koi ponds.” If you could ask a koi, however, it would disagree! A koi pond is one that properly addresses the needs of the koi, no matter what construction or filtration style the owner desires. A koi pond is first about the needs of the fish and second about the aesthetic desires of the owner.

The needs of koi must also be addressed within the boundaries of the customer’s budget and the profit margin of the professional pond builder. To address these needs, one must have a good working knowledge of both. I believe lack of knowledge, coupled with the customer’s budget and the profit motives of the pond builder, are the main reasons our industry has such a bad reputation. As I’ve stated in a previous article: with no codes, specs or rules, 80 percent of ponds fail within the first year of construction, and 80 percent of those fail within the first six months. Our industry needs to clean up its act, and that starts with knowledge and ends with integrity.

Basic Requirements

I don’t profess to understand all the needs of koi, but I think I know enough to build them a good habitat. On a basic level, koi need a few things that promote a healthy aquatic environment. This environment consists of a minimum volume of water per adult fish, good water quality, a high dissolved oxygen content, good circulation and a healthy diet. It’s also important to keep dangerous obstructions like sharp rock stands and large, freestanding lights out of the pond so the fish don’t damage themselves.

These parameters are arguable and variable depending on geography, temperature and overall water quality. But in general, we can agree on some basic numbers. Minimum water volume per adult koi, for instance, can vary, with 1,000 to 2,000 gallons on the high end for those who intend to raise show quality fish, to as low as 250 to 500 gallons for those with backyard ponds. The water volume per koi also depends on the circulation, biofiltration, depth, dissolved oxygen content, quality of food, et cetera. Permanent overcrowding lowers water quality and requires a tremendous increase in biofiltration and dissolved oxygen. Mother Nature will create a fish health issue in overcrowded ponds, and she can be very creative as to how she implements the die-off when you break her rules.

more than a koi pond article men digging
Most under-gravel ponds accumulate sludge
that is time-consuming to remove

Source: Pondtrade Mag

Depth and Volume

The smaller in volume or shallower a pond is, the more circulation, filtration and oxygen per gallon that will be required for the pond to function well. Volume and depth are your friend. Smaller ponds tend to be overstocked and shallow ponds are dangerous for the fish in terms of safety from predators and temperature control.

Our industry suffers heavily from what I call “shallow pond syndrome.” The 18-inch to 24-inch depth that some call “standard” was created by some manufacturers early on to avoid safety codes created for the pool industry. This allowed anyone to become a pond builder, which helped with sales … but it was not what the fish needed. Deep water keeps fish safer from predators, and fish like deeper water where they can exercise and use their swim bladder more effectively. Even goldfish like deep water. Koi like to feed from the surface by going vertical, so a koi over 18 inches can’t feed naturally in a shallow pond.

Furthermore, the sunlight affects 100 percent of the water in a pond to a depth of 18 to 24 inches. Deeper water gives the system a cushion of water volume relatively unaffected by sunlight without increasing the footprint of the pond. Deep ponds have a more stable temperature and don’t fluctuate as much between night and day or when the air temperatures change suddenly with changes in weather.

Three feet seems to be an industry agreed minimum in the koi world, and I regularly build ponds that are five to six feet deep. Ponds over 18 to 24 inches require a pool-coded gate and fence, lighting and egress in the form of steps to climb out. These are all safety issues and are easy to design and build into the project.

Quality and Clarity

Water quality and water clarity are two different things. Water quality is what you do for your fish and water clarity is what you do for you. Just because the water is murky doesn’t mean it’s unhealthy for your fish … but why spend all the time and money on a koi pond and not be able to see and enjoy what you’ve created? A clear pond can also be unhealthy, so don’t confuse clarity for good water quality. Fish eat, excrete and consume oxygen. I consider myself a “fish poop management specialist” and design pond systems from this perspective. When fish eat, they produce ammonia and solid waste. A koi pond is a decorative wastewater treatment plant, and your job is to design a system to act as such.

Two basic types of bacteria (nitrosomonas and nitrobacter) colonize koi ponds, establishing the nitrogen cycle which converts the ammonia into nitrites and then into nitrates. A mature pond system should test zero ammonia, zero nitrites and, in some cases, zero nitrates. Nitrates are usually removed with water changes. Plants consume nitrates and in a very mature, highly oxygenated system, biofilters can house a less-understood family of bacteria that also consume nitrates. Be cautious with a too-heavily planted pond for a given water volume because plants produce oxygen during the day and consume oxygen at night, causing stress to the koi. My pond is 12 years old, has 7,400 gallons and houses 25 large adult koi, with no plants and zero nitrates. I’ve built several ponds that fall into this category. I think I know why they don’t produce nitrates, but not in any way I could prove or describe.

Solids must be removed from the system in real time to prevent anaerobic decomposition in the pond. Decomposition produces acids and toxins that are unhealthy for fish. The filtration system should consist of two phases: pre-filtration (the removal of heavy solids from the water column) and biofiltration (the nitrogen cycle). Each of these can be accomplished in over half a dozen ways that all work well. They don’t work well in every combination, however, so choosing a pre-filtration method that couples well with a particular biofiltration type in a specific style of pond is the challenge.

The Bottom Matters

Most of the dedicated koi pond owners and builders, including myself, have been opposed to rock- and gravel-bottom ponds and tend to build instead with full-flow bottom drains, mid-water drains and skimmers on a clean, unobstructed pond surface. This type of construction requires out-of-pond pre-filtration that is easier to clean and maintain. Traditional rock and gravel ponds, where the water is pulled from the bottom of the rock layer through slotted piping and sent to the biofilter, work well for a short time before they become overwhelmed. The rock layer and slotted pipe become restricted and serve as a place for solids to become trapped. Without good circulation the system goes anaerobic.

The biggest problem is cleaning them. The pond has to be completely drained and the sludge-filled gravel cleaned and put back in place with all the fish removed to quarantine during the process. The “once a year cleaning” is expensive — and it’s never really enough. Coupled with the movement of the fish, which is always hazardous and stressful for them, this becomes a lessthan- desirable approach to pond design, construction and maintenance. It’s often one type of failure in the long list of why ponds fail. If you are building a pond that produces anaerobic sludge that must be removed in this way, you are part of the problem.

Recently a new type of “under-gravel” rock bottom pond the industry is calling the “ecosystem pond” has been promoted that uses a much more extensive “aerated” suction grid system with back-flush capabilities. This method is much more complicated to construct but doesn’t lead to the type of anaerobic sludge buildup we’ve seen with the original systems.


Biofiltration consists of two basic categories: non-aerated trapping filters and aerated bio-reactors. Non-aerated biofilters are the most common, and when operated at the proper flow rate for the size of filter, they convert the ammonia and trap fine particles for water clarity. Aerated biofiltration does a huge volume of ammonia conversion because of the high dissolved oxygen content but won’t trap fine particles for water clarity. The caution here is that aerated biofilters have a much higher flow rate but must be used in conjunction with a fines trapping filter or excellent pre-filtration in order to maintain good water clarity.

The majority of biofilter manufacturers overrate their equipment or use a “once every two hour” turnover rate calculation in their marketing. Unlike water gardens, however, koi ponds generally need to have a turnover rate of at least once an hour. Turnover rate is defined as the number of times the total volume of pond water is sent through filtration and back to the pond in one hour. When choosing a biofilter, make sure you’re applying the right flow rate to the right piece of equipment for a given situation. A good example is the pressurized filter market. If a specific manufacturer states a capacity enough for a 10,000- gallon pond, that’s usually at a two-hour turnover rate. That means that at a one-hour or 45-minute turnover rate, it should flow between 3,500 and 5,000 gallons per hour at a maximum.


Good circulation and high dissolved oxygen content are important, so try not to create shapes in the pond construction that trap debris, and install current jets where necessary to promote good flow characteristics. Install air diffusers on timers, and don’t think that a waterfall is always going to be enough oxygen for the pond. Both koi and the bacterial colonies in the biofiltration system consume a huge amount of the dissolved oxygen, so add aeration. Run part of the system with air-lifts whenever possible. Air-lift pumping systems move the water and add air with the same energy, keeping the pond consistently oxygenated in real time. Open biofiltration exposes more of the pond’s surface area to oxygen, while pressurized filters consume oxygen without adding any.

more than a kio pond img 3


Feed a high-quality food with a high natural protein content derived from a source the fish might actually have access to. I feed a 49 percent protein food with no fillers. This keeps my koi and my biofilters healthier. A koi’s natural diet does not include wheat germ, chicken feather meal, soy, corn or algae. Koi do eat worms, rotifers, mollusks and the critters that live in the algae you think they are eating. All of these have a protein content of over 50 percent, and koi graze all the time.

On a final note, a koi’s head should not be the widest part of its body. A healthy koi will expand out behind the gill plates and have a nice, curved body shape. Follow the requirements I’ve outlined above, and soon all of your koi will start showing some healthy curves!

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