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.