How to Determine GPH Water Flow

Blue fish swimming in an aquarium

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When you purchase equipment like water pumps and filters for your saltwater aquarium, they typically come with a gph (gallons per hour) rating. However, there are sure to be sources of resistance that reduce the rated flow. For example, the flow may be slowed by a power filter or canister filter, a filtering sponge, or head pressure from pumping water uphill. Testing the water flow after the equipment is installed gives you a more accurate picture of the actual flow rate.

Proper water flow, tank water turnover time, and water movement in an aquarium are essential. Once you have determined the actual gph output you are getting from your equipment, use the flow rate and the water capacity of your aquarium to calculate how many times per hour your tank water is being turned over. A healthy tank water turnover rate to strive for is six to 10 times per hour. Many aquarists feel that higher turnover is better, especially for a reef tank system.

Determine GPH Water Flow Rate

  1. Prepare a gallon-size milk container (or a similar container) so it is clean. It should be sterile if you are going to dump the water back into the tank after the test.
  2. Turn off the pump and attach a short length of clear plastic tubing to its outflow nozzle. The tubing should fit snugly over the nozzle. Insert the other end of the tubing into the one-gallon container.
  3. Turn on the pump and, using a stopwatch, time how long it takes for the container to fill, then turn off the pump. Write down this time. For example, consider your pump is able to fill the gallon container within 15 seconds.
  4. Divide the timed rate by 60 to find the gallon per minute rate: 60 divided by 15 equals 4.
  5. Multiply the gallon per minute rate by 60 to find the gph rate: 4 times 60 equals 240 gph.
  6. Don't forget to disconnect the tubing and restart the pump so it is back into use for the aquarium.

Calculate Your Tank's Turnover Rate

  1. Measure the height, width, and depth of your aquarium in inches to determine the amount of water you have in your tank. Measure 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.
  2. Use an online tank water volume calculator to determine the amount of water in the tank. Alternatively, you can convert the measured dimensions into gallons by multiplying the measured height, width, and depth to find the volume in cubic inches. Divide the result by 1,728 to convert to cubic feet. Multiply the result by 7.5 (the number of gallons in a cubic foot).
  3. Divide the calculated gph rate by the tank water volume to find the turnover rate. For example, if your tank actually holds 38 gallons and the flow rate is 240 gph: 240 divided by 38 is 6.32. The pump system filters all of the tank water 6.32 times per hour.

Choosing a New Pump

If you are purchasing a new pump and want to estimate what the flow rate will be before you buy it, take the manufacturer's flow rate and divide that into your measured tank water volume. This will give you how many times per hour the water pump will turn over the tank water, without any resistance factors.

For example, if you are looking at a water pump that is rated at 400 gph, and you have a total of 55 gallons of actual tank water, the pump will turn over the tank water 7.27 times per hour (400 divided by 55 equals 7.27) before filters and other factors reduce the flow.