Determining the size of the aquarium pump you need to get can seem like a fairly easy decision, but it’s actually not that easy as you might think. It is A complex that will determine your aquarium set-up. Getting a big pump can potentially overflow your tank, while a very small pump can cause a lack of water flow the tank needs. To decide what size pump to get you need to see how much linear drain space your tank has and what is the maximum flow rate the tank can handle. To calculate linear drain space, you’ll need to measure all sides of the overflow that are in direct contact with water in the aquarium (usually it’s two sides for traditional corner overflow and three for traditional back wall overflow). Add up the side measurements and you’ll get the total linear drain space. Then all you have to do is compare the calculated linear drain space with the required linear drain space you need for your drain size. To see what’s the maximum flow rate of your tank, you will need to see how many tanks drain your aquarium has and what size bulkheads are used in each drain. As a general rule of thumb that people follow, you can see below the size drains that can accommodate the flow: For a bulkhead size of ½” and a maximum flow rate up to 150/gph, the recommended overflow size of at least 2” of linear drain space For a bulkhead size of ¾” and a maximum flow rate up to 325/gph, the recommended overflow size of at least 5” of linear drain space For a bulkhead size of 1” and a maximum flow rate up to 600/gph, the recommended overflow size of at least 9” of linear drain space For a bulkhead size of 1.5” and a maximum flow rate up to 1300/gph, the recommended overflow size of at least 20” of linear drain space For a bulkhead size of 2” and a maximum ax flow rate up to 2300/gph, the recommended overflow size of at least 35” of linear drain space