Why an ATO Is Essential
Water evaporates from reef tanks constantly — only pure water evaporates, so salt stays behind and salinity rises. Miss a day of manual top-offs, especially in summer, and salinity creeps up. An ATO (Auto Top-Off) detects the water level drop and pumps in RODI water automatically, keeping salinity rock-solid.
How They Work
Optical sensors detect water level using light refraction — more reliable than float switches and less prone to salt creep issues. Float switches are cheaper but can get stuck. The better systems include safety features: maximum run time limits, secondary high-water sensors, and alarm notifications.
What to Buy
Budget (£20–40): Jebao/Jecod ATO units — basic but functional, single optical sensor
Mid-range (£50–100):
- Tunze Osmolator 3155 (~£80–100) — The gold standard. Dual sensor system with optical + float backup. A common recommendation for most reefers
- AutoAqua Smart ATO — Compact with built-in pump
Premium (£100+): Neptune Apex ATO, XP Aqua Duetto
Evaporation Rates (UK Conditions)
- Winter: 1–3 litres/day for a 200L open-top tank
- Summer: 3–6+ litres/day
A 300L system in a warm room can go through 20–25L per week in summer.
Common Mistakes
- Using saltwater in the ATO — Only ever use RODI water
- No failsafe — A stuck sensor can overflow the sump and flood the room
- Ignoring the reservoir — An empty reservoir means no top-off and rising salinity
Browse ATO systems from UK sellers — Tunze Osmolators are built to last, making used ones a smart buy.
What ATO system are you using? Any tips for reliability?
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