How to Fix an Aquarium Heater Making a Knocking Noise

Aquarium heaters work by heating the water in your aquarium tank, thus making your pet fish happy and healthy.

However, many people don’t often consider the impact of water and its source on their fish before using it in their tanks. The water type and flow can affect your aquarium heater’s performance over time, and this can cause it to make a knocking noise.

Most water sources contain dissolved minerals, sands, pebbles, and other soil deposits that will eventually settle down at the bottom of the tank.

Your aquarium heater can also make a knocking noise if the lower clamp on the heater is too low.

You also need to unplug your heater, remove it and make sure water is not getting inside it.

You can fix the knocking heater by thoroughly flushing out the sediments from your tank until the aquarium heater is free from deposits, moving the lower suction cup up the heater, or replacing the heater if it is getting water inside it.

The reason for moving the lower suction cup up on the heater is to move it farther from the heating element.

Aquarium Heater

When the lower suction cup is too low, water that is trapped between the suction cup clamp and the glass of the heater is heated by the heating element which causes it to expand and is forced out which causes the knocking sound over and over.

Although other factors like a damaged anode rod and gas valve can cause the heater to make a knocking noise, the accumulation of sediments in the heater or the lower clamp being too low are the principal reasons most aquarium heaters make noise.

Water Source And Heater Performance

Most aquarium owners fill their tanks with tap water and other water sources containing many dissolved mineral deposits. This type of water is called hard water.

Whenever you use hard water in your aquarium tank, its mineral contents like magnesium and calcium will settle at the tank bottom because the heating action of your aquarium heater will separate the water from the mineral.

These deposits will continue to build up until the water trapped under them start to make a knocking noise when heated.

Besides, the hard water surrounds the heater heating element with heat-conducting mineral deposits, which cause the heater element to work harder and make noise.

How to Prevent Your Heater from Making A Knocking noise

Hearing a knocking noise from your heater implies that the heater is not working efficiently to heat the aquarium water.

Any time you work on your aquarium heater, make sure it is unplugged.

If you have made sure the lower clamp holding the heater is not too low and your heater is still knocking, here are some other ways to get rid of the knocking:

  • Examine the anode rod: The aquarium heater has a metal anode rod that protects your tank against rusting and corrosion. A knocking noise may result if the current anode rod is old and not functioning. Therefore you should try to replace it.
  • Fix the heater rubber diaphragm: A loose rubber diaphragm can cause a knocking noise by vibrating against your aquarium tank. Thus, you need to check the diaphragm and tighten it if it is loose.
  • Check the gas valve: This is crucial, especially if using a gas heater in your aquarium. A non-functioning gas valve will geometrically increase the aquarium’s internal temperature and cause your heater to make a knocking noise.

If you have examined and fixed your heater’s anode rod, gas valve, and rubber diaphragm and still hear the knocking noise, you need to flush your tank.

How Many Times Should You Flush Your Aquarium Heater

It is good if you flush your aquarium heater at least once every year using any viable method. If your aquarium heater has a heater blanket, you need to remove this cover to perform the flushing process effectively.

How to Flush Your Aquarium Heater

Flushing is essential whenever your aquarium heater is making a knocking noise because this shows that the heater is struggling to heat the tank water efficiently. Before you start the flushing process, you need to put on gloves to avoid burning yourself.

You can flush your aquarium heater by yourself by following the steps below:

  • Turn off the power supply to your aquarium heater: You must prevent electric current from entering the heater before flushing the heater. This is important because the passage of electricity within the heater without enough water in the tank will expose the heater to dry air and damage the beater.
  • Set the gas supply to pilot mode: You need to regulate the gas supply by setting it to vacation or pilot mode if you use a gas heater.
  • Turn off the cold-water valve: You should turn off the cold water supply to achieve excellent results from the flushing process.
  • Open a hot water faucet: You need to open a hot water faucet to prevent vacuum formation within the pipes. The hot water faucet will ensure adequate pressure release during the flushing process.
  • Connect a garden hose to the drainage valve: You need a garden hose that serves as a passage for the water during the draining stage. The garden hose may lead into a large bucket or outside your house.
  • Drain the tank: You should now switch on the drainage spigot to properly drain the tank. This process will allow the aquarium water to flow through the garden hose. You may notice that the water released during this process is colored, showing the passage out of some deposited sediment.
  • Flush the aquarium heater: You need to switch on the cold water valve to allow the cold water supply into the tank. You should leave the water to flow for about 15 minutes or leave it on until the tank is filled halfway with cold water. You can repeat the flushing process until the water flowing out from the garden hose is colorless and devoid of sediments.

If you have carried out the above flushing process, your aquarium heater should work perfectly without making noise again. However, suppose your aquarium heater still makes a knocking noise after flushing. In that case, you need to check your aquarium tank water pressure or get a new heater. You should get a new aquarium heater if the sediment has clogged the drain valve.


Your functioning aquarium heater can make slight noise due to the usual expansion and contraction within the tank.

Still, a loud knocking noise is something to worry about. You can fix this knocking noise by flushing your heater and replacing the gas valve or moving the lower heater clamp higher away from the heating element.

You should also make sure there is no water getting inside the heater by simply unplugging it, taking it out, and making sure there is no water in it.

Even though flushing the heater once a year may eliminate the knocking noise, it would help if you target the hard water itself since its mineral content is the cause of sediment deposition.

You can make your hard water soft by installing water-softening equipment.


Leave a Comment

Your email address will not be published. Required fields are marked *

Scroll to Top