This is one of the most common questions pond owners ask, and there are no simple answers.
Most recommendations that you'll hear involve formulas expressed in terms of inches of fish per 100 or 1000 gallons. Following that formula without some modifications can be risky. Consider that one 24" koi and 12 two inch koi both constitute 24 inches of fish, yet the one large koi weighs at least 10 times as much as the 12 small fish and will therefore need to consume 10 times the amount of food to thrive. The amount of food going into the pond and therefore the amount of waste coming out of the fish is the biggest contributor to poor water quality, mostly in the form of excessive ammonia and nitrite readings.
Most recommendations that you'll hear involve formulas expressed in terms of inches of fish per 100 or 1000 gallons. Following that formula without some modifications can be risky. Consider that one 24" koi and 12 two inch koi both constitute 24 inches of fish, yet the one large koi weighs at least 10 times as much as the 12 small fish and will therefore need to consume 10 times the amount of food to thrive. The amount of food going into the pond and therefore the amount of waste coming out of the fish is the biggest contributor to poor water quality, mostly in the form of excessive ammonia and nitrite readings.
So in order to allow for this huge difference in weight per length of fish, we will divide the sizes of koi into small, medium, and large. If we assume small koi or goldfish are 3 to 6 inches, medium koi are 6 to 12 inches and large koi are 12 to 24 inches, recommendations for inches of fish per gallon make a little more sense.
We also need to consider what type of pond and what type of pond keeper we're dealing with. Is it a "natural" pond with little or no filtration, or one with the latest advanced filtration? Is the hobbyist diligent about water changes and filter maintenance? Does the pond have adequate circulation? Is it in the sun most of the day? Do they want to grow and maintain show fish, or are they more interested in goldfish? Also realize that koi will grow quickly, and a reasonably stocked pond will become overcrowded soon.
So here are my recommendations:
Small Fish (3-6 inches) 10 to 15 fish per 1000 gallons
We also need to consider what type of pond and what type of pond keeper we're dealing with. Is it a "natural" pond with little or no filtration, or one with the latest advanced filtration? Is the hobbyist diligent about water changes and filter maintenance? Does the pond have adequate circulation? Is it in the sun most of the day? Do they want to grow and maintain show fish, or are they more interested in goldfish? Also realize that koi will grow quickly, and a reasonably stocked pond will become overcrowded soon.
So here are my recommendations:
Small Fish (3-6 inches) 10 to 15 fish per 1000 gallons
Medium Fish (6-12 inches) 5 to 10 fish per 1000 gallons
Large Fish (12-24 inches) 1 to 3 fish per 1000 gallons
Given that there are so many variables involved, realize that these guidelines are very general and should be used as only as a starting point in discussions with your customer.
Randy LeFever
Do the fish won't grow if they are over crowded?
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