Just as there are fish the world over, in every possible environment, there are fish that can thrive in almost any pond, providing the conditions are healthy and the fish are well matched. Some exceptions are extremely cold regions where deep water freezes completely solid. Koi and goldfish can actually survive water at 32 degrees Fahrenheit, so long as there is good quality liquid water with adequate oxygen. At the other extreme, fish can survive in water as warm as 85 degrees Fahrenheit, which is at the high end of comfy for tropical fish. This upper limit is because water can hold very little oxygen at higher temperatures, and all fish need to breathe.
Within those extremes, the key to survival is choosing the right fish for the pond and ensuring the environment is as beneficial as possible. People who are sick, starving and injured won’t survive well in even a moderately cold environment and fish swim by the same rules. If you live in a climate that frequently gets below freezing and you want fish to overwinter in your pond, you’ll want to design it so that it won’t freeze solid. Make it deep -- at least 18” or as much as 30” in extremely cold regions. Choose fish accustomed to cooler temperatures like the traditional koi and goldfish, as well as beautiful pumpkinseeds, fascinating archerfish and golden rainbow trout. Your local pond supplier can suggest even more options. In very warm environments, where the temperatures go no lower than 65 degrees Fahrenheit at night and rise above 72 during the day, tropical aquarium fish can survive quite happily. Some good candidates are Plecos (algae eaters), Mollies, Guppies, Platies and Swordtails. For the in-between temperate regions, cold water fish are consistently well suited, although making some allowances for heat waves (sun shades, buckets of ice, etc.) is an excellent idea.
