What size pipe should I use?
Pipe size is extremely important on koi ponds. Pool builders will often use smaller diameter pipe when building a swimming pool because it is cheaper and easier to work with. They can easily overcome the restriction caused by the smaller pipe by using large, high pressure pool pumps. This is acceptable because they usually only run these pumps 4 to 6 hours a day. Koi ponds, on the other hand, must move large volumes of water 24 hours a day. The smaller you make your pipe, the more pressure it will require to move water through it.
Pumps are rated by how much water (gallons per minute) they can move at a certain amount of head pressure. 1 foot of head pressure is the amount of pressure required to push water up vertically 1 foot. Example: To move 20 gallons of water per minute through 50 feet of 1 inch pipe would require 11.4 feet of head pressure. If you increased your pipe size from 1 inch to 2 inch your head pressure would drop to 0.4 feet of head. The lower you keep your head pressure, the smaller, more efficient pump you can use. This can save you a lot of money over the years.