By Stephen M. Meyer
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.
What Do We Do?
Q. My husband and I have a dilemma on our hands and we really need help. We have two pacus (Colossoma macropomum), approximately 4 years old, living in a 125-gallon aquarium. I'm sure you can appreciate how inappropriate and utterly inadequate this setup is considering the size of these fish and their growth rates. Of course, we now know that our purchase of pacus was incredibly stupid, but the store clerk assured us that the fish would grow to the size of their environment and no more.
The folks we have talked to locally all admit to turning their problem pacus loose in a large local river (which accounts for those summer reports of piranhas being found in our area) or having a big fish fry. Both of these seem irresponsible to us, and we'd like to come up with something better.
We have priced custom acrylic aquariums and found the price to be staggering. So we come to our dilemma: What do you with these gentle giants?
We think a pond is good solution because the price of setting one up is more within our means and we can increase the size as needed. My husband is concerned with several things. Just how big can these fish get? We have heard that they can run 60 pounds or more. How big would our pond have to be? What about heating? Our climate is fairly harsh in winter.
Would it be easier to try to relocate these fish to a more natural environment? If so, where? How long can these hardy guys live, anyway? We're in our 30s — is it possible that they could attain that age, too?
No one in our area is willing or able to give us any ideas, and most people think we are just plain silly to be worrying about two big old fish. What do you think?
A. Colossoma macropomum is commonly referred to as the black-finned pacu. It is a native of the Amazon and a vegetarian. As you discovered, it can grow to be a big fish — over a foot in length if properly cared for. I think you might expect them to reach 10 pounds or so under confined conditions. Although your tank can hold the pair, as you suspect, it is far from optimum.
Pacus evolved in Amazon wetlands where tree roots, emergent and submergent plants, and woody material provide an extensive network of protection. With this in mind you could greatly increase the "habitability" of the 125-gallon tank by incorporating vegetative and woody structures that simulate the complex environment of a tropical river — leaving some large open areas for swimming.
A move to an outdoor pond — heavily vegetated with submerged plants — would be a superior solution. Pacus are used to water temperatures in excess of 70 degrees Fahrenheit, and would certainly flourish in a nice 2000-gallon pond during the summer, but successful overwintering is unlikely at best. Trying to keep the water in the mid- to upper 60s would be very expensive.
Setting the animals loose is just not an option. As your letter implied, releasing non-native species is an ecologically irresponsible thing to do. Most tropical species won't survive our winters in most parts of the country, but even a brief existence in the wild is sufficient to transmit devastating diseases and parasites into local populations of native fish. Exotic cast-offs that do survive usually decimate native fish in by predation or, in the case of your pacus, out-competing native fish for vegetation.
In my view, aquarists who release fish into their local streams or ponds do far more damage to wetlands systems than developers and polluting industries. It seems to me that it is a fundamental responsibility of any aquarist or pondkeeper who tires of a pet, or finds himself unable to properly care for the animal, to find it a proper home. This means searching out other hobbyists or a commercial source willing to take the animal — for free if need be. Call some aquarium societies in your region. See if some pet store or commercial facility might want the pacus for a lobby display tank.