Auto Top-Off (ATO) for your aquarium: a quick guide to calculate how much water you’ll need while you’re away and how to choose the right reservoir next to the sump.
Why ATO matters when you’re on vacation
Evaporation never stops—especially when you’re not at home. A well-planned ATO keeps salinity stable and helps prevent stress and potential damage to fish and corals. In short, a reliable ATO can literally save your holiday.
How much water do you really need? Do the math
Before you leave, measure your average daily RO/DI consumption. Watch the ATO reservoir for 3–5 days and note the drop in water level.
Example:
If your tank uses 1.2 liters/day and you’ll be away for 10 days, you’ll need at least 12 liters of RO/DI water prepared in advance.
Pro tip: Always add a 20% safety margin.
Which reservoir should you use?
You have two options:
- Keep spare jerrycans
Prepare 2–3 canisters (5–10 liters each) and ask a trusted friend/family member to top up the ATO reservoir when the level drops.
Simple, but requires someone reliable. - Use a larger reservoir by the sump
If you run an ATO with pump and sensor, swap the reservoir for a bigger one. A 25–30 L canister or a square plexiglass tank (ideally with level marks) can cover more than two weeks.
Mind the reservoir height relative to the sump and the pump.
How to avoid ATO issues
- Test the system for at least three consecutive days before leaving.
- Make sure the ATO sensor is clean and correctly positioned.
- Check for debris in the reservoir and tubing.
- Don’t wing it: if possible, ask someone to check every 4–5 days or use a webcam for quick remote checks.
Extra: print and leave instructions. Laminate a quick sheet and place it next to the sump listing:
- Where the spare canister is stored
- Where to pour the water
- How often to top up (if you don’t use an automatic ATO) or when the main reservoir is smaller.
Clear instructions can be the difference between a calm return and a disaster.
A well-planned auto top-off can be the difference between a stable system and a struggling one. Whether you’re a curious beginner or an experienced reefer, planning for evaporation is something you should never underestimate.
What ATO solution do you use? Tell us in the comments or reach us on Telegram, Instagram, Facebook, X/Twitter and YouTube. Need help? Join our forum.
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