Q. Fifteen years ago my family and I built a 20-by-10-foot garden pond that is 1 foot deep. The goal was to see if the pond could be maintenance-free, with zero fiddling. The concrete and wire pond is filled with well water. There’s a 1050 gallon-per-hour fountain that only runs a few hours each week, and about 1000 pounds of plants — mostly irises, lilies, hyacinth, elodea, vinca and parrot weed. The pond is also home to an ever-increasing collection of fancy guppies, goldfish, and koi. The goldfish and koi breed so often that we have to give most of them away. We don’t even feed the fish.
Our yearly maintenance schedule consists of removing 50 percent of the plants every spring and fall. The plants are not in pots, never receive any special care except thinning, and we do not do water changes.
My question to you is: Why would a pondkeeper go to all the trouble of changing water and filtering when all you need to do to keep the pH, nitrate, nitrite, ammonia and so on in check is toss in some plants? The fish stay in the pond during winter, and the pond freezes over for 34 weeks at a time, with no apparent ill effects on the fish.
We recently had a 100,000-gallon pond dug in the front yard and are having it lined with an agricultural reservoir liner ($10,000). What would the appropriate fish load for this pond be? We plan to have a small waterfall opposite the island. We will probably add about 100 small koi, 200 goldfish and 200 gallons of mosquito fish in. The pond receives direct sun 10 hours per day in summer and six in winter. We will again be using lots of plants.
A. Thanks for a great letter, with great pictures. Your description of the vegetative filtration (plants) in your pond should give many readers something to think about.
I would, however, suggest that weekly water changes are still a good idea, even with your system. Vegetation does a great job of removing nitrogen, phosphorus, minerals and metals. However, over time other substances build up in the water — such as proteinaceous substances from fish food and wastes, and tannins from decaying leaves — that plants do not remove. Some of these may have important negative effects on water quality and fish health, and removing them is only possible with water changes.
Water changes do not just remove pollutants. The addition of new water “freshens” the aquatic environment of the pond, adding back beneficial substances and micro-nutrients removed by chemical and biological activity. For example, maintaining a stable pH over a long period of time will require water changes to compensate for the neutralization (removal) of alkalinity. Highly acidic bog ponds are loaded with vegetation, but with a pH around 4 they also have no fish.
Water replacement is fundamental to natural pond ecology — almost all ponds have some influx of fresh water. Some are fed by brooks and streams, others by underground springs, and some that appear isolated are actually fed by surface runoff that flows across a watershed over land, bring minerals and nutrients to the pond.
Why isn’t rainfall sufficient? Well, it may be in many respects, but it is unlikely that rainwater will contain all the necessary minerals and ions needed to completely rejuvenate the pond.
Now, on to your monster pond. You noted on the back of the photo that the pond was about 100 by 45 feet, and about 5 feet deep. Based on the kidney-like shape, the surface area is about 3000 square feet. Assuming no biological filter system, my basic rule of thumb is one 12-inch koi per 100 square feet. Thus, my recommendation it is for 30 koi, plus or minus.
Many readers will probably find such a low number to be unreasonable, but keep in mind that the biologically useful area for ponds is really the first 2 feet or so. In terms of fish load, we are really interested in the top 40,000 gallons. If you replicate the massive scale of vegetation used in your smaller pond you will have some additional fish stocking potential but have less water surface area and volume.
From what I can see in the photo you did not leave much pond area for shallow vegetation zones (depths of six to12 inches). The typical 12-inch shelf around the perimeter of average-size ponds (2500 gallons or so) is simply not adequate to give emergent plants the extensive area needed to display them clustered in full glory. Given the huge size of your ornamental pond, however, you have a great opportunity to create a serious pond landscape plan with a real ecological foundation.