Bubbles in Your Aquarium? It's Probably Your Auto Top-Off (ATO) System

Living Oceans

If you've noticed bubbles coming from the return nozzle or hear a gurgling noise from your Living Oceans Aquarium, don't panic. This is one of the most common and easiest issues to resolve.

In most cases, bubbles are simply a sign that your Auto Top-Off (ATO) reservoir has run empty. As water naturally evaporates from the aquarium, the ATO system automatically replaces the lost water to maintain a stable water level and salinity. When the reservoir runs out of water, the return pump begins drawing in air, creating bubbles in the aquarium.

Fortunately, refilling the ATO reservoir only takes a few minutes, and the system will automatically restore the correct water level.

Watch the video below to see how to refill the ATO reservoir and restore normal operation. The guide underneath provides additional troubleshooting tips, maintenance advice and answers to common questions.

Why Is the Auto Top-Off (ATO) System Important?

The Auto Top-Off (ATO) system is designed to automatically replace water lost through evaporation, helping to maintain a stable water level and salinity in your Living Oceans Aquarium.

Maintaining a stable salinity is essential for the health of your fish, corals and beneficial bacteria. If the ATO reservoir runs empty, evaporated water is no longer replaced. This causes the water level in the return pump chamber to drop, allowing the return pump to draw in air and gradually increasing the aquarium's salinity if left uncorrected.

Fortunately, this is one of the simplest maintenance tasks and only takes a few minutes to resolve.

Signs Your ATO Reservoir Needs Refilling

As water naturally evaporates from your aquarium, the ATO system replaces the lost water from the reservoir inside the cabinet. Once the reservoir becomes empty, the return pump chamber can begin drawing in air instead of water.

You may notice one or more of the following:

  • Bubbles entering the aquarium from the return nozzle.
  • Gurgling or slurping noises from the return pump.
  • A lower water level in the return pump chamber.
  • An empty ATO reservoir.
  • The ATO sensor displaying a flashing red light.

These are normal indicators that it's time to refill the ATO reservoir.

How to Refill the ATO Reservoir

Refilling the reservoir only takes a few minutes.

  1. Turn off the power to the aquarium.
  2. Remove the ATO reservoir from the cabinet, or leave it in place if it's easier to refill where it is.
  3. Fill the reservoir with fresh water. Filtered water is preferred, although tap water is suitable for replacing evaporation.
  4. Fill the reservoir to approximately 15 litres, which typically provides 4–6 weeks of operation, depending on your home's temperature and evaporation rate.
  5. Feed the tubing back through the opening in the reservoir lid.
  6. Place the ATO pump back on the bottom of the reservoir and reconnect the hose.
  7. Slide the reservoir back into the cabinet and replace the lid.
  8. Turn the aquarium power back on.

What Happens Next?

After restoring power, you may briefly see more bubbles entering the aquarium. This is completely normal.

When the aquarium is turned back on, the ATO system will detect the lower water level in the return pump chamber and automatically begin topping it up. Once the correct water level has been restored, the bubbles will disappear and the return pump will operate normally again.

Allow a few minutes for the system to complete this process before checking that everything has returned to normal.

Understanding the ATO Sensor

The water level sensor in the return pump chamber indicates whether the ATO system is operating correctly.

  • Solid green – The water level is correct and the system is operating normally.
  • Flashing green – The ATO pump is actively topping up the water.
  • Flashing red – The ATO system requires attention, usually because the reservoir has run empty or the system needs resetting.

If the sensor continues to flash red after refilling the reservoir:

  1. Turn the aquarium power off.
  2. Wait approximately 3 seconds.
  3. Turn the power back on.

The sensor should reset and return to normal operation once the correct water level has been restored.

Checking the ATO Sensor Position

The ATO sensor is factory positioned using an external alignment magnet on the back of the aquarium.

Do not move this magnet, as changing its position may prevent the ATO system from maintaining the correct water level. If you believe the sensor has been moved or damaged, please contact the Living Oceans Support Team before attempting to adjust it.

If the ATO Is Still Not Working

If the reservoir has been refilled and the sensor reset but the system is still not topping up correctly, check the following:

  • Confirm the ATO pump is sitting on the bottom of the reservoir.
  • Check that the tubing is securely connected and not kinked.
  • Inspect the pump intake for any debris or blockage.
  • Ensure all electrical connections are securely plugged in.

If the problem persists, please contact the Living Oceans Support Team.

Routine Maintenance

The ATO system requires very little maintenance. To keep it operating reliably, we recommend:

  • Checking the reservoir water level weekly.
  • Refilling the reservoir before it runs completely empty.
  • Keeping the ATO sensor clean and free from algae or salt deposits.
  • Periodically checking that the pump and tubing remain free from blockages.
  • Confirming the ATO pump remains positioned on the bottom of the reservoir.

Frequently Asked Questions

How often will I need to refill the ATO reservoir?

A full 15-litre reservoir typically lasts 4–6 weeks, depending on room temperature, humidity and seasonal evaporation.

Can I use tap water?

It is recommended to use filtered tap water and tap water conditioner to ensure it is safe.

Why are bubbles coming from the return nozzle?

This usually means the return pump chamber has become too low because the ATO reservoir is empty. Once the reservoir is refilled, the ATO system will automatically restore the correct water level and the bubbles should stop within a few minutes.

Why is my ATO sensor flashing red?

A flashing red light usually indicates the reservoir has run empty or the system requires resetting. Refill the reservoir and then switch the aquarium off for three seconds before turning it back on.

Need More Help?

If you've completed the troubleshooting steps and the issue persists, please contact the Living Oceans Support Team.

To help us diagnose the issue as quickly as possible, please include:

  • Your aquarium model (80L, 130L or 200L).
  • A brief description of the issue.
  • The colour of the ATO sensor light.
  • Photos or a short video showing the ATO reservoir, return pump chamber and sensor.

This information will help us provide the fastest and most accurate support.