What size pump do you need for a 165 gal fish pond?

For most ponds with fish you need to circulate the total volume of the pond once per hour. So for a 165 gallon pond, you theoretically need at least a 165 GPH (gallon per hour) pump. However, not all pumps are created equal, and the flow rate of pumps is usually rated at zero head pressure and with nothing attached. So a 165 GPH pump on the bottom of a 2 foot deep pond with a few feet of tubing and maybe a water feature or waterfall could reduce that 165 GPH to perhaps 100 GPH or less. So for a 165 gallon pond, you would probably want to look at 250-350 GPH pumps. If you have an above average fish load or a large waterfall or stream attached, you may want to go even larger.