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How to choose
Evaporation removes fresh water and raises salinity, sometimes by a lot in a single day. An ATO replaces that water with RO/DI automatically, keeping salinity stable. Many reefers call it the first automation worth buying. Redundancy matters, because a stuck sensor can flood a tank with fresh water.
- Use a small reservoir so a failure can't overdose fresh water.
- Clean optical sensors monthly.
- Never top off with saltwater. Salt doesn't evaporate.
For the science behind it, read our Salinity and RO/DI guide.
How we review: we paraphrase what owners report in retailer reviews and forum threads, plus picks and test results from BRStv, Reef2Reef discussions and published reef chemists. We only show a rating when we verified it at the source, with the date we checked. We don't invent scores or quotes. Prices and stock change, so check the retailer.
Well-regarded picks
Tunze Osmolator 3
Optical sensor + thermal safety sensor
- What reviews say: strengths
- Owners report fast setup, solid build and fewer cords than older models. One replaced an 8-year-old Osmolator with the new one.
- Weaknesses
- The pump may not fit through a 5-gallon jug opening, it has no siphon break, and one owner replaced the optical sensor yearly.
- Who it's for
- Sump systems where redundancy and long track record matter.
- Expert consensus
- BRS names the Osmolator its most reliable ATO because two different sensor technologies back each other up, and calls it its most popular ATO. It topped BRS's 2024 ATO best-seller list.
Sources
Where to buy:
Tunze Osmolator 3 Nano
Single optical sensor, for tanks up to about 66 gal
- What reviews say: strengths
- Easy setup, quiet operation and reliable level control. Owners upgrading from older Tunze units are happy.
- Weaknesses
- The sensor is large for very small tanks, and the bright indicator light bothers some owners.
- Who it's for
- Nano and AIO tanks.
- Expert consensus
- Tunze rates it for systems up to about 250 L (66 US gal), with a single optical sensor accurate to 0.5 mm.
Where to buy:
XP Aqua Duetto2
Dual optical sensor, compact
- What reviews say: strengths
- A tiny sensor, plug-and-play setup and very precise level control. Many owners report months to years of trouble-free running.
- Weaknesses
- Several owners report sensor or controller failures within about two years, or pumps needing replacement, and some say the older generation was more reliable.
- Who it's for
- AIO and nano tanks with tight rear chambers.
- Expert consensus
- BRS names it the best ATO for nano aquariums, and it was a top-three ATO seller in 2024.
Sources
Where to buy:
Red Sea ReefATO+
Smart ATO with app
- What reviews say: strengths
- App integration with other Red Sea gear, and a temperature probe. Some owners report flawless operation.
- Weaknesses
- Reviews are polarized. Several owners report sensor failures and overfills, and it needs frequent sensor cleaning. It doesn't work in freshwater.
- Who it's for
- Owners already running Red Sea's ReefBeat ecosystem.
- Expert consensus
- It was a top-three ATO seller at BRS in 2024. We found no independent long-term test.
Where to buy:
Spot an error or a product we should add? Specs and availability change often, so always check the manufacturer's current listing.