How to Determine GPH Water Flow

Tips On Calculating Gallons Per Hour Water Flow for Saltwater Aquariums

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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 the 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 in 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 turnover 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 turnover rate to strive for is 6-10 times per hour. Many aquarists feel that more is better, especially for a reef tank system.

Steps to Determine Gallons Per Hour Water Flow Rate

  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.
  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 bottomĀ 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.
  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.
  1. 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 stopwatch 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.

Determine Gallons Per Minute

  • Take the time in seconds it took to fill the one-gallon container and divide that into 60 seconds to get the number of gallons it pumped per minute.
  • Our powerhead example: 60 seconds divided into 15 seconds = 4 gallons per minute.

Determine Gallons Per Hour (GPH)

  • Now multiply the number of gallons per minute by 60 minutes and you get the number of gallons per hour pumped.
  • Our powerhead example: 60 minutes multiplied by 4 gallons = 240 GPH (gallons per hour).

Determine How Many Times Per Hour the Tank Water Volume is Turned Over

  • Divide the number of gallons per hour by the actual gallons of tank water volume you got from Step 2.
  • Our powerhead example: 240 GPH (gallons per hour) divided into 38 gallons = 6.32 times per hour our tank water passes through the powerhead.

If you are purchasing a new piece of equipment and want to estimate what the flow rate will be before you buy it, take the manufacturers flow rate and divide that into the actual tank water volume you got from Step 2. This will give you how many times per hour the water pump will turn over the tank water, without any resistance factors of course.

  • Example: You are looking at a water pump that is rated at 400 GPH. You have a total of 55 gallons of actual tank water. This water pump will turn over the tank water 7.27 times per hour. Estimate in any factors for resistance and decide if this pump is going to be sufficient for your needs.

That's it plain and simple. These 7 steps can be applied to any piece of equipment to determine water flow rates.