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Reef maintenance guide

Auto Top-Off: Maintenance and Failure-Proofing

An auto top-off keeps salinity steady, until it sticks on or runs dry. How ATOs fail, the failsafes worth having and the simple maintenance that prevents both.

Updated 13 October 2026FailsafesFree routine

An auto top-off fails in two ways: it sticks on and overfills the tank with fresh water, or it stops and lets salinity creep up. Bulk Reef Supply calls overfilling the biggest risk. Use an ATO with a second sensor or a maximum run time, keep the sensor clean, refill the reservoir before it runs dry and test the failsafe every few months.

How ATOs fail

Bulk Reef Supply describes the two failures.

  • Overfill. The sensor or controller doesn’t switch the pump off, and fresh water keeps flowing. Salinity drops fast. Bulk Reef Supply calls this the biggest risk.
  • Underfill. The sensor misses a drop, or the reservoir runs dry. Salinity creeps up as water evaporates.

Failsafes worth having

Bulk Reef Supply recommends redundancy: a second sensor a little above the first that cuts the pump if the first fails, a maximum run time, or an audible alarm. A reservoir that holds only a few days’ top-off also limits how much fresh water can go in if everything else fails.

Maintenance

Bulk Reef Supply says sensors most often fail because of algae on them, or a snail or crab sitting on them.

  • Wipe the sensor clean every few weeks
  • Check nothing is living on or near it
  • Refill the reservoir with RODI water before it runs dry
  • Clean the reservoir now and then
  • Test the failsafe: lift the main sensor out or hold the float, and check the pump stops

Before a holiday

Bulk Reef Supply’s holiday advice: install an ATO at least a couple of weeks before you go, and make sure it has run reliably for 5–7 days. See going on holiday.

Common questions

Can I put saltwater in my ATO?

No. Top-off replaces evaporated fresh water. Use RODI water only.

What’s the most reliable ATO sensor?

Any type can fail. A second sensor or run-time limit matters more than the type.

How often should I clean my ATO sensor?

Every few weeks, or whenever you see algae on it.

Sources and limits

Last checked 13 October 2026. Follow your ATO maker’s instructions for cleaning sensors and testing failsafes.

Keep going

Add ATO care to your routine · Free Reef Companion · All calculators