This is absolutely one of the first questions we inevitably get from new pond owners. Rules of Thumb are great to put you in the ballpark but they really should not be taken as the end all last decision. I hear this DANGEROUS old RULE OF THUMB in the aquarium hobby brought over to the pond world on occasion.
How many koi fish per gallon of water?
If you are consulting with a high-end Koi keeper you will hear rules like one female Koi for every 1000- gallons of water in your pond or one male Koi for every 500 gallons of pond water. This may seem radically conservative to the person that just spend a ton of money installing a 3000-gallon pond in their backyard. That translates to 3 female Koi or 6 male Koi for that 3000-gallon pond.
If you are starting with small Koi remember they will get big and it really doesn’t take that long. In just two or three years you’ll have a hefty size herd of Koi mobbing around your pond. In my travels, I’ve come across a great deal of over crowded ponds with unhappy Koi. One of the worst I’ve seen was a 350-gallon pond with 13 very large adult Koi. It’s not pretty! In fact, it was a rescue mission. Don’t put yourself or your Koi in that position.
Here’s a fun fact to help you wrap your arms around the demands of large Koi. A 13″ Koi weights approximately 1 pounds. Guess what a 26″ Koi tips the scales at. Approximately 13lbs! When Koi double in size you’re taking a look at the tenfold increase in body mass! That’s an amazing burden on a pond’s life support system!
I can provide you a rule of thumb for you in the ballpark but I can not express to you enough, the value of a good life support system for your pond. Now go do the math in your own pond and tell me how your Koi Herd piles up to this rule of thumb. One Koi per 250 gallons of well filtered and preserved Pond Water.