By Stephen M. Meyer
Q. We have a cement-lined stone pond about 6 feet long by 4 feet wide by 2.5 feet deep (1.8 x 1.2 x 0.8 meters). We use well water with a pH of about 7.4. The pond has a 4-foot high waterfall that is driven by a submersible pump. The pond contains six ordinary goldfish, each about 4 to 6 inches (10 to 15 centimeters) in length. Our problem is that the water becomes murky and overloaded with algae. We drain the pond and scrub out the algae, but in almost no time it becomes murky again. How can we control the algae?
A. The murkiness in your water is suspended algae. Depending on the mix of algae, the nutrient levels, the amount of sunlight that hits the water and the time of year, the water may look deep green, greenish-brown or brownish-black.
The problem of controlling suspended algae in fish ponds is probably raised more often than any other question. I am sure it will not surprise you to learn that a comprehensive answer would take up many pages. Let me offer some straightforward solutions.
Algae are plants, and they require three things to grow: water, light and nutrients (phosphorus, in particular). Because the water is a given, let us consider the other two factors.
Your water evidently has a high nutrient content. The best way to starve out the suspended algae is to put other plants in the water that will compete for the nutrients. Water hyacinth, iris, parrots feather and any submerged oxygenating plants are all good choices.
Most importantly, I would stop scrubbing down the pond walls. The algae growing on the sides is not the same algae that is making the water murky. If you let the algae on the walls grow, it will help starve out the suspended algae. It is also a good food source for your goldfish.
Another way to cut down the nutrients is to install a biological filter. I estimate your pond holds about 360 to 400 gallons (about 1500 liters). There are many options for installing a small biofilter, but an easy solution is to use one of the foam-sleeved Danner (Supreme) Universal Pond filters. This just attaches to the inlet side of your submersible pump. It will remove suspended debris from the water and become a biological filter after a month or two. All you need to do is gently rinse out the foam sleeve every week.
You also might consider cutting down the sunlight reaching the pond. Many people in your area of the country install a sunscreen or awnings. Others use wood lathing or some type of screening material.
Alternatively, if your pond receives six or more hours of sunlight daily you could grow water lilies. I would try to cover about 70 percent of the pond's surface with them.
Affordable Filters
Q. In another issue of AFI you mentioned that it was possible to build a filtration system for a pond for around $30. Was that a misprint? Can you give me a list of materials and instructions?
A. No, it wasn't a misprint. There are perhaps dozens of filter designs available for small ponds that can be bought or built for around $30 — not including the pump. Commercial examples include the foam-sleeve filters made by Danner Mfg. (the Supreme line) and the sponge filters offered by Tetra, among others.
A 20-gallon (76-liter) plastic pail plumbed with PVC pipe and filled with ½-inch (1.3-centimeter) gravel or lava rock will serve a pond of around 500 gallons (1893 liters) or so and cost about $30 (see accompanying diagram). It can be installed either inside or outside the pond. You can find the details of numerous designs discussed in publications such as KOI USA. A local koi, goldfish or pond club can provide a wealth of information on inexpensive but effective filters.
MixingGoldfish
Q. I recently brought home a feeder goldfish for a pet. He's small and is in a 2-gallon (7.6-liter) fish bowl. I went to the pet store to buy another fish, but all they had were small orandas and other show goldfish. The dealer said I should not mix feeder goldfish with show goldfish, but never explained why.
A There are two reasons why goldfish hobbyists do not recommend mixing feeder goldfish and show (fancy) goldfish. The first is that feeders tend to be fast, frisky swimmers. They are lean and well equipped for swimming. Fancy goldfish, in contrast, tend to be slow, ungainly, fat and not well equipped to compete for food. Thus, if you mix them in the same tank, your feeders will get most of the food and grow rapidly, but your expensive fancy goldfish will barely get enough to stay alive.
The second reason is that all goldfish mix quite well — in fact, too well. The feeders are close approximations of wild goldfish. The fancy goldfish are distant genetic strains. If the two breed, the result will be a regression to wild goldfish lacking the color, form and finnage that make fancy goldfish so special and expensive. Moreover, because the feeders are far more vigorous than fancy goldfish, there is a good chance that breeding will result in serious physical harm to the fancy goldfish.
Goldfish in Ponds
Q. This summer I will be enlarging my garden pond. The new pond will be 7 feet long, 5 feet wide and 3 feet deep (2.1 x 1.5 x 0.9 meters). I would like to keep fancy goldfish. The pond will not have a filter or aeration system. How many fancy goldfish can I keep? Will they be able to overwinter in this pond?
A. I estimate that your new pond will have a surface area around 24 square feet (2.2 square meters) and a volume of 630 gallons (2385 liters), assuming an oval shape. Fancy goldfish vary considerably in size, so I can only suggest a guideline.
Because you will not have either a filter or aeration system, I would not put more than 10 small- to medium-sized fancy goldfish in that pond. If the fish are closer to 6 inches (15 centimeters), I would put no more than three in the pond. This is based on a rule of thumb: No more than 100 grams of fish mass per 1000 liters of water in unfiltered, unaerated, ponds.
The pond should be well stocked with aquatic plants. I would monitor ammonia levels very carefully during the first month.
Goldfish overwinter quite nicely in ponds in New England that are 3 feet deep if, and only if 1) the stocking level is very low, as I have suggested, 2) the pond is absolutely clean of leaves and debris before it freezes over and 3) the fish are healthy and well fed going into the winter season. If you can keep a small hole open in the ice — using a de-icer, for example — you can decrease the stress of overwintering on the fish.
Fancy goldfish are more sensitive to the cold than comets. I have kept thousands of comets outdoors and never lost one. Orandas tend to do well also, but long-finned varieties often get bacterial infections in the extreme parts of their fins. I would not keep a prized fish outdoors during the winter in New England.