The Big Fish Challenge: Transitioning from water gardens to koi ponds

web_conrad_main_koi I don’t build or sell equipment for the pond trade. I am one of the people who raise the fish for which the ponds are built. During the last decade, I have seen a steady decline in demand for goldfish with a concurrent increase in demand for koi. That reality changes everything we think we know about water gardens. Only a few of us have the resources to have grand champion koi, but almost all of us can maintain truly beautiful fish. All it takes is a few changes in the way we design and operate our ponds. Koi are big fish and they need a completely different environment than goldfish to thrive.

Kurt Kleinholz carefully moves a parent koi to a new pond.
Kurt Kleinholz carefully moves a parent koi to a new pond.

Getting Started

For the best results, koi ponds should be at least three feet deep. While goldfish can do well in shallow ponds that receive little or no added food, koi in such an environment lose luster and body condition. But, when koi get sufficient food for optimum appearance, the waste load overwhelms most water garden filter systems. Koi need deeper water and more filtration than goldfish. Most of the goldfish in water gardens are less than eight inches long and weigh two to four ounces. Water garden koi, on the other hand, are often between 15 and 20 inches long and weigh two to five pounds.

This article contains recommendations to water gardeners for adapting filtration systems in existing ponds to accommodate the rising popularity of koi. My recommendations are based on my research on filter systems for ornamental ponds at Langston University and on the bioreactor systems I have developed for holding and display tanks at our own farm.

If you have or obtain a client who wants to change the focus of her pond from goldfish to koi, or who has koi but doesn’t like the maintenance they require, there are some relatively easy ways to help her. Garden pond filter designs have advanced rapidly during the past few years. Most are now capable of maintaining small koi or small numbers of larger koi. But what can you do if you get a referral for a client with an older garden pond and filtration system? You probably got the call because the client is frustrated by the inability of his current filtration system to maintain clear water, or by the amount of maintenance required to keep clear water. During the initial consultation, be sure to find out if he is satisfied with his current fish population or if he wants more and/or bigger fish.

Table 2. Comparison of surface area (ft2) per volume (ft3), and cost per foot2 of various types of commercially available filtration media for use in ornamental ponds. Prices are in US dollars during 2013. The data indicate the amount of media needed to transform nutrients to nontoxic components. Up to 10 times the indicated amount of media may be needed to achieve acceptable water clarity.
Table 2. Comparison of surface area (ft2) per volume (ft3), and cost per foot2 of various types of commercially available filtration media for use in ornamental ponds. Prices are in US dollars during 2013. The data indicate the amount of media needed to transform nutrients to nontoxic components. Up to 10 times the indicated amount of media may be needed to achieve acceptable water clarity.
Table 1. Total body length (TL: tip of nose to tip of tail) for koi of various sizes(1) and the quantity of media (feet3 or gallons) needed to maintain acceptable water quality for a single fish of the sizes indicated.
Table 1. Total body length (TL: tip of nose to tip of tail) for koi of various sizes(1) and the quantity of media (feet3 or gallons) needed to maintain acceptable water quality for a single fish of the sizes indicated.

Once you have determined the client’s desires for the pond, check Table 1( see images) to find the amount of filtration media needed to maintain his target koi population, and then go to Table 2 to find the amount of media surface area in the current filtration system. The table compares different media based on surface area and cost per foot2. These two tables will give you an evaluation beyond the simple “good for ponds up to” information. In order to obtain and maintain water clarity, you must know what the filter really does, and the tables are intended to reduce callbacks and maintenance headaches for you and your clients.

As an example, we see from Table 1 that an 18-inch koi should weigh about 3.5 pounds, and that it will need 0.2 feet3 of media that contains 200 feet2 of surface area per foot3 of media volume. If the pond has 10 fish of that size, then the media requirement becomes two feet3, or 14 gallons. Comparing the media requirement to 10 goldfish, if we assume the fish are eight inches long and have weights equal to koi up to this size, we now have a requirement for only 0.1 foot3, or 1.2 gallons of media. If the existing filter vaults are too small to contain the needed media, it is easy to see how filtration systems become overwhelmed.

We can easily deepen an established rock-lined pond to accommodate koi by removing the rocks below the plant shelf and using them in landscaping around the pond. This simple fix can increase the usable depth and volume of a pond by 50 to 100 percent. Koi ponds don’t have rock liners because the rocks interfere with water filtration. A single koi will produce as much waste material as one to two dozen goldfish, and that waste is much easier to manage without rocks. Deeper ponds don’t heat or cool as quickly as shallow ponds, so their water quality tends to be more stable, but deeper water does require active aeration with air stones or diffuser disks. Deeper ponds are easier to keep clean because we can collect waste from the bottom rather than with a surface skimmer.

Adding bottom drains allows us to remove plumbing from the pond and switch from submersible to external pumps. While external pumps are more expensive, they cost less to operate, and when operating cost is included external pumps win — especially after the first year. Within the three-year normal life of a submersible pump, electrical savings will pay for a new external pump. Add a strainer basket on the intake side and they are also easier to keep clean, especially if you are accustomed to putting a submersible pump inside a skimmer box.

You can get a retrofitted bottom drain for an existing pond and connect it to the skimmer vault. Install a check valve in the line so the pump won’t lose prime. Remove the submersible pump from the skimmer vault and install a horizontal perforated plate at the water level to collect litter. You or your client can easily remove leaves with a net. If the pond has a big skimmer vault, consider retrofitting it as a filter. Add an air stone or dome and fill it 60 percent full of moving bed media. You can remove the solids more effectively downstream of the pump. Install a vault for solids removal and plumb it as an upflow unit. Put an air stone or disk on the bottom for agitation to dislodge solids during flushing. Add media like Springflo or Bio-Balls. They can capture solids and are easy to clean without using a lot of water. Mats that are fine enough to trap solids are a pain to clean. They use both time and water in excess of their usefulness in solids removal versus clogging. If you can’t live without filter pads, at least change the way you use them. Install the most open mesh nearest to the water intake, with progressively denser mesh closest to the pond return. You will spend less than half the usual time for pad maintenance.

If the existing filter system needs more surface area, the cheapest solution may be to remove the old media and replace it with new media that has larger surface area and volume. If there is not enough volume for more media in the existing skimmer, filter and waterfall vaults, you can add new vaults. You will probably need to do the bioreactor remodeling over a period of four to eight weeks. Replace media in only one vault at a time so that existing bioreactor activity is not lost. Check N (both total ammonia and nitrite) levels and wait until they stabilize to replace the next media vault. If addition of bacterial cultures does not maintain healthy N levels, you may have to turn off the UV and let algae help with the N removal until the new bioreactor matures.

Planktonic algae are still best managed with UV systems. Ionic systems are best suited for water features without fish. Dissolved copper can reach harmful or toxic concentrations in lined ponds. Contractors can make these suggested changes within a single day. They will allow you to make your clients’ fish and ponds much more attractive and probably earn you many referrals. I hope they also serve as a call of opportunity to manufacturers to provide larger, more efficient filtration systems.

Stay tuned for Part 2 – Filtration Systems, in the November/December issue.

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