Determining what size pump to purchase may seem like a fairly easy decision. However, it is one of the more complex and biggest decisions you will need to make when setting up an aquarium with a sump.
If you buy too big of a return pump, you potentially overflow your tank or sump. In that case, you are also spending more money on a bigger return pump than you need. Alternatively, if you purchase too small of a return pump, you may lack the flow required for your skimmer, filter sock, refugium, filter media reactor, heater, chiller and/or other equipment to function properly.
So how do you decide what you need? To get started you’ll need to answer the following 2 questions:
- How much linear drain space does your tank have?
- What is the maximum flow rate my tank drains can handle?
How to Calculate Linear Drain Space
- Measure all sides of the overflow that are in direct contact with aquarium water; a traditional corner overflow will have 2 sides in direct contact with the aquarium water and a traditional back wall overflow will have 3 sides in direct contact with the aquarium water
- Add side measurements together to calculate total linear drain space. Please see an illustration below.
- Total Linear Drain Space for 2-Sided Corner Overflow = A + B
- Total Linear Drain Space for 3-Sided Back Wall Overflow = 1 + 2 + 3
- Compare the calculated linear drain space against required linear drain space needed for your drain size in the chart below and ensure it meets or exceeds the recommended linear drain size. If your overflow meets or exceeds the minimum linear drain size, your maximum flow rate will be determined by the size and number of drains as shown in the next section. If the overflow is less than the minimum linear drain size, please contact your aquarium manufacturer for the recommended flow rate of your specific overflow.
How to Determine Maximum Flow Rates for Gravity Feed Drains*
This is determined based on 2 factors:
- How many tank drains your aquarium has
- What size bulkheads are used in each drain
As a general rule of thumb, the following size drains can accommodate the following amount of flow:
| Bulkhead Size | Max Flow Rate | Recommended Overflow Size |
| ½” | up to 150/gph | at least 2” of linear drain space |
| ¾” | up to 325/gph | at least 5” of linear drain space |
| 1” | up to 600/gph | at least 9” of linear drain space |
| 1.5” | up to 1300/gph | at least 20” of linear drain space |
| 2” | up to 2300/gph | at least 35” of linear drain space |
How to Choose the Right Pump Size for My Aquarium
So how do you determine what size pump is right for your aquarium? As a general rule, you want to choose a pump that can move water at or near the maximum flow rates that your drains can handle. When making your determination, you will want to make sure to account for any head pressure on the return line. Here are a couple examples below…
Example Set-up #1:
- Tank Size: 75 Gallons
- Overflow: 5” x 5” 2-Sided Corner Overflow (10” of linear drain space)
- Drain: 1x 1” Bulkhead
- Head Pressure: 5 feet
Recommended Pump Size: 1 Drain x 600 gph = Approximately 600 gph @ 5 feet of head pressure
Example Set-up #2:
- Tank Size: 225 Gallons
- Overflow: 1x 5” x 8” x 5” 3-Sided Back Overflow (18” of linear drain space)
- Drain: 2x 1” Bulkhead
- Head Pressure: 8 feet
Recommended Pump Size: 2 Drains x 600 gph each = Approximately 1200 gph @ 8 feet of head pressure
* Flow rate through a siphon drain will generally be significantly higher than a gravity drain.