ATO Reservoir Sizing & Evaporation Calculator
An auto top-off (ATO) is only as useful as its reservoir is large. Run out mid-day and your salinity creeps up; run out for three days while you're traveling and you can crash sensitive corals. This calculator sizes the reservoir for your real-world evap rate, with a 25% headroom buffer for summer spikes.
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Cap the tank, watch the water-level drop over 24 h. Open-top reefs with strong lights and a skimmer can hit 1 gal/day per 50 gal.
How long can your tank go without you refilling the ATO reservoir? 3 days is a sensible minimum; 7+ for travel.
ATO reservoir sizing
- Minimum capacity
- 1.5 gal
- Recommended (25% headroom)
- 1.9 gal
For exactly 3 days at the current evap rate.
Evap spikes in summer or under new lights. Buy bigger than you think you need.
Don't forget: if you dose kalkwasser through the ATO, your reservoir water is high-pH and shouldn't sit longer than ~2 weeks before it reacts with atmospheric CO₂.
How to measure evaporation accurately.Mark the water line on your sump (or display) at midnight. Don't top off for 24 hours. Mark again. The drop, times your tank's water-column footprint, is your daily evap. Open-top tanks with high light and skimmer venturi can evaporate twice as much as covered tanks — measure your real setup.