Please banish from your mind any thoughts of the crowded tanks you can see at the pet store. Unless you maintain an industrial capacity filter system and powerful pumps, that level of density is not for you. Fish need room to swim, oxygen to breathe, and a volume of water adequate to maintain a healthy environment with a constant influx of fish waste. In a normal pond environment, there are several factors that should be considered in calculating your fish population, but you can start with figuring the surface area of your pond. An extra deep pond doesn’t give you any extra credit, in this part. You’ll want to plan on no more than 1 inch of fish length per square foot of pond surface. If you’re keeping koi, it’s a good idea to calculate the pond volume as well, and allow at least 35 gallons per inch of koi or 20 gallons per inch of any other common fish. This means that a 2000 gallon pond can support about 5 adult koi OR about 100 goldfish. Keep in mind that your fish will continue to grow, so you need to allow for the adult size of your fish or you’ll be faced with some hard choices in a very short time.
If you do find yourself overpopulated, you can give some of your prized pets away to a trustworthy friend, you can upgrade your filtration system to process higher amounts of fish waste, or you can install a larger pond. That last option is always a temptation, but you may eventually run out of yard. In the end, the best policy is always to add new fish slowly, giving your pond’s chemistry time to balance out before you add more. Make allowances in case some of your fish tend toward giant-size, and remind yourself frequently that sometimes, less is more.
