Instructions
Perform a casual inspection of the device to look for any simple problems. Make sure the filter is plugged in and getting power, that there aren't any cracks in it or sludge built up, and that there are no leaks in any of the hoses.
Ensure the filter is submerged enough in the water so the impeller on the lower-right side is completely covered. If the filter is not deep enough in the water, proper filtration cannot occur.
Rinse out the filter with aquarium water to see if it is clean enough to continue filtering. Use aquarium water to prevent chemical additives like chlorine from getting into the filter, since they are toxic to fish. If the filter is old or dirty, replace the filter.
Check the impeller to make sure the blades inside it can move freely. Grains of dirt or gravel can get in the chamber sometimes and block the blades from spinning, which will negatively affect filtration.
Inspect the motor. If everything else is correct and the motor is still not running, it may have shorted out and require replacement.
Aquaclear Filter Troubleshooting
Aquaclear is a popular brand of filtration for aquariums. Aquariums require filters to keep the water moving throughout the tank and to remove decaying fish food, excrement and other debris. This keeps the water clearer and more pleasant to look at, but it also keeps the water much healthier for the fish. If the filter starts to malfunction, look into it immediately. Troubleshooting an Aquaclear filter takes a little time.