Some aquarium filters tell you right on the box their intended capacity. However, plenty of types of aquarium equipment lacks such guidelines, and you have to figure it out on your own. You can calculate the needed filter flow for your aquarium based on the aquarium's volume.
Measure the dimensions of your aquarium in inches. You can do it in centimeters, but you will have to convert them to inches for the next step. If you measure in centimeters, divide by 2.54 to get your measurements in inches.
Find the volume of your aquarium. To calculate volume, multiply the length, height and depth. This will give you a number in cubic centimeters. You will need to covert this number to gallons or liters. To do this, multiply your answer by 0.00433 to find gallons or 0.016387 to find liters.
Multiply your volume by 6. This is the desired filter flow rate for an aquarium. You want to purchase a filter that can run six times the volume of your aquarium water through itself per hour. Some extremely specialized tanks -- like nano reef aquariums -- require much more powerful filtration. For specialized tanks, do additional research.
Calculate the filter's GPH if the box does not list them. To do this, hook a length of tubing to the filter's inlet, and let the filter's outlet flow into a container with a known volume. Turn on the filter and time it to see how long it takes to fill the container. Convert this number to gallons and figure out how many gallons per hour that translates to.
- Filter manufacturers sometimes rate their filters by tank capacity. However, the "gallons per hour times 6" rule works more consistently.
- Some filters need to be primed or filled with water before you plug them in.
- Follow all manufacturers' instructions; a risk of shock is extant, since most aquariums combine water and electrical componentry.
- Never submerge a filter that's not designed to work outside the tank.
- Never start a filter dry. This can burn out the moving parts.