Fish vary in their appetites and the settings in which they thrive. So before stocking up, assess your situation. How warm does the pond get in summer? Will it freeze in winter? Does it maintain high oxygen levels—through the action of a pump, natural stream, plant metabolism, or wind—or tend toward boggy oxygen deficiency? Do aquatic plants such as lotuses or water lilies filter nutrients generated by decomposition below the surface, or do nutrients accumulate in the water? In other words, what are the living conditions for fish?
Fathead minnows, native to the Northeast, tolerate low oxygen levels and high nutrient levels; at maturity, they measure just an inch or two from gills to tail. In larger ponds, they serve as a vital food source for predators, including bass and bluefish; in large and small ponds alike, their appetite for mosquitoes, insect larvae, and algae helps keep populations in balance. The bottom-dwelling Johnny darter, which thrives on small snails and insect larvae, requires cool temperatures and high water quality. The fecund and voracious mosquito fish—native to the southeastern United States—has a name to warm the cockles of any gardener’s heart, but it can wreak havoc in ecosystems where it is not native. In California, its diet of tadpoles and larvae has been blamed for the decline of native frogs and newts.
While ornamental exotics such as goldfish and Japanese koi boast attractive colors—and prices to match—native species tend to be less vulnerable to wading birds, reducing the need for protective netting or artificial decoys. For native color in the southern pond, consider the pygmy killifish, golden topminnow, or banded pygmy sunfish, all miniature insectivores with a taste for mosquitoes and midges. Where aquatic weeds have gotten out of hand, triploid sterile grass carp—native to China—can put plants in their place without muddying the waters, but many states require a permit for their release.
By mixing and matching fish and plants, gardeners can establish a complex and resilient ecosystem that requires only limited maintenance. On Cape Cod, for example, scientists concerned about the spread of West Nile virus and equine encephalitis have called on the insectivorous banded sunfish—a striking, 3-inch-long native of the Atlantic coast from Maine to Florida—to reduce mosquito populations in cranberry bogs, protecting the health of workers and tourists. For guidance on selecting species native to your region, contact your local cooperative extension, department of natural resources, or fisheries department. They may be able to provide lists of recommended species or even free consultations from experts in the field.