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How to Determine GPH Water Flow Rates


When you purchase equipment for your saltwater aquarium, like water pumps and filters, most of the time they come with a GPH (gallon per hour) rating. Sure the box says you'll get 300 GPH, but what about sources of resistance that lessen the flow rate. For example, are you using a Power Filter that is preceded by a pre-filter sponge? How about head pressure that a water pump will encounter if you need to pump water uphill? Are you using a Canister Filter with carbon in the media chamber, or that has a pre-filter pleated cartridge or filtering sponge? These are just a few things that can contribute to a lower water flow rate.

What if you purchased or received a piece of equipment that had no box, paperwork or product identification markings? For our water flow rate test in this article, we are using a Hagen AquaClear Power Head 402 that we have on our 55 gallon aquarium for many years. This powerhead has no physical markings for identification on it, so if it were not for the box that it came in, we would not know what brand name it is, so we feel it is an appropriate choice for our test.

Proper water flow, tank water turn over time and water movement in an aquarium are essential. By using these 7 simple steps you can determine the actual GPH output you are getting from your equipment, as well as find out how many times per hour your tank water is being turned over.


    A healthy tank water turn over rate to strive for is 6-10 times per hour. Many aquarists feel that more is better, especially for a reef tank system.

Step 1

    Get a container, a one gallon size works well. Be sure it is clean and sterile if you are going to put the water back into the tank when done.

Step 2 - Determine Actual Tank Water Volume

    You need to figure out how many actual gallons of water you have in your tank. To do this you measure the height, width and depth of your aquarium in inches, but only the area where water touches the glass. Do not measure the area where your substrate covers the bottom, or the space at the top of the tank where there is no water. Once you've done this you can use the Tank Water Volume Calculator from the Aquatics by Design site to figure out how much water is in your tank by gallons. If you have a lot of live rock and coral in your tank, this lessens the amount of actual water volume in the tank. You can adjust the water volume by deducting an estimate of how much in gallons of water you think your live rock and coral take up. Our 55 gallon tank has 38 gallons of actual water volume in it.

Step 3

    Turn off the piece of equipment you want to test. For the test on our powerhead we attached an eight inch piece of clear, soft tubing the same diameter as the outflow nozzle to direct the water into the one gallon container.

Step 4

    To determine gallons per second, turn on the piece of equipment and time out how long it takes for the container to fill up, turning off the piece of equipment when the container is full. For timing, use a stop watch or a watch that has a second hand. Write down this time and restart the piece of equipment back into use on the aquarium. Filling the one gallon container with the water pumped through our powerhead, it took 15 seconds.


    For turning on and off the piece of equipment to fill the container and do the timing, this can be a much easier job with two people. One person for each task.

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