Best ATO for a nano reef tank
An auto top-off (ATO) keeps a nano's salinity steady by replacing evaporated water with fresh water. ATOs aren't tied to one tank model, so instead of a fit check we compare what each maker states about overfill protection, the pump, the sensor mount and the reservoir.
Quick answer: the AutoAqua Smart ATO Duo is our top pick: it has a backup (failsafe) sensor plus a run-time alarm and comes with its pump, and its magnet mount is rated for glass up to 12.7 mm. Like every unit here, it doesn't include a reservoir.
How we rank
First, the number of overfill safeguards the maker states: a backup (failsafe) sensor and a run-time limit, two at most. A run-time limit that stops the pump ranks ahead of one that only sounds an alarm. Then, units that include a pump rank ahead of ones that need a pump bought separately. Controller dimensions and reservoirs are listed but not ranked: no maker here publishes controller dimensions, and none includes a reservoir.
AutoAqua Smart ATO Duo
- backup (failsafe) sensor
- run-time alarm
- pump included
A failsafe sensor above the ATO level sensor, plus QST: alerts when a refill takes 6× longer than the first, detects sensor or controller malfunction and an empty reservoir; 72-hour non-filling reminder.
Pump: 74 gph (280 lph), 8.2 ft (250 cm) max head. Mount: Magnetic, for glass up to 1/2 in (12.7 mm); universal tube holder sold separately; siphon break and 6.5 ft (200 cm) tubing included. Maker source
Every verified ATO, compared
| ATO | Sensors | Overfill protection | Pump | Max glass | Reservoir | Controller size |
| Smart ATO Duo AutoAqua | 2 sensors, optical | Backup sensor + Run-time alarm | Included | 12.7 mm | Not included | not published |
| Osmolator nano 3152 Tunze | 1 sensor, type not stated | Run-time cutoff | Included | not confirmed | Not included | not published |
| Smart ATO Micro AutoAqua | 1 sensor, optical | Run-time alarm | Included | 12.7 mm | Not included | not published |
| Smart Level Hydor | type not stated | Backup sensor | Not included | 10 mm | Not included | not published |
- run-time limit stops the pump
- pump included
- one overfill safeguard, not two
Dosing-time monitor stops the pump after 1.4 min of continuous running (factory setting, tanks under 100 L / 26 US gal; a jumper sets 2.45 min for 100-200 L), then pulses it every 10 s until power-cycled.
Pump: No flow rate stated; about 0.42 US gal (1.6 L) per run at 0.80 m head, max lift about 2 m. Mount: Magnet holder for the sensor. Maker says: Nano or small aquariums up to 200 L (55 US gal).
Maximum glass thickness left out: the manual says 12 mm, but a conflicting 10 mm figure turned up in search results we could not open. Power supply 5012.010 (wattage not stated). Sensing principle not named.
- run-time alarm
- pump included
- one overfill safeguard, not two
Quick Security Technology (QST): audible and visual alert when a refill takes 3× longer than the first refill (the first refill is not timed).
Pump: 74 gph (280 lph), 8.2 ft (250 cm) max head. Mount: Wet-side and dry-side magnets for glass up to 1/2 in (12.7 mm); universal tube holder sold separately; siphon break and 6.5 ft (200 cm) tubing included.
Power input 100-240 VAC 50-60 Hz. 1-year warranty.
- backup (failsafe) sensor
- one overfill safeguard, not two
- pump not included
A separate ALARM sensor blocks top-up and sounds an alarm.
Pump: Not included; socket for a refill pump up to 50 W. Mount: Magnet suction-cup support included, for glass up to 10 mm; 2 m cable; adhesive cable clamp.
Hydor's page listed it out of stock on 2026-10-19. Sensor count, sensing principle, voltage and controller power are not stated.
FAQ
Do I need an ATO on a nano reef?
You don't strictly need one, but it helps more on a nano than on a big tank. Evaporation takes out fresh water and leaves the salt behind, and on a small volume the salinity rises fast between top-offs. An auto top-off (ATO) replaces the evaporated water with fresh RO/DI water as it goes, so salinity stays steady. Top off with fresh water only, never saltwater.
Single sensor or dual sensor: which ATO is safer?
The main risk with any ATO is a stuck sensor or pump that keeps adding fresh water. Makers guard against that in two ways: a second, higher sensor that stops top-off if the water reaches it, and a run-time limit that stops the pump or sounds an alarm if a refill runs too long. We rank units with both safeguards first. Whatever you buy, keep the reservoir small enough that emptying all of it into the tank would not swing salinity dangerously.
Does an ATO fit any nano tank?
Mostly, yes: ATOs are not tank-specific. Check two things. The sensor mounts with a magnet, and each maker rates it for a maximum glass thickness (listed in the table), so compare that with your glass. And on an all-in-one tank the sensor usually goes in the rear chamber where the return pump sits, since that is where evaporation shows up, so make sure there is room for it there. None of these makers publishes the controller's dimensions.
Does an ATO come with a water reservoir?
None of the units here lists a reservoir as included, so budget for one. Tunze recommends its own storage tank for the Osmolator nano. The Hydor Smart Level also has no pump: it switches a refill pump you buy separately (up to 50 W).
More for nano reefs
Best nano reef lights · Best nano protein skimmers · Wavemakers for the Coralife BioCube 32 (LED) · Wavemakers for the Fluval Evo 13.5 · Reef heaters
Gear by tank: Coralife BioCube 32 (LED) · Fluval Evo 13.5 · Fluval Flex 15 (57L) · Fluval Flex 32.5 (123L) · Waterbox Cube 20 (AIO) · Red Sea MAX Nano G2 Cube · Innovative Marine Nuvo Fusion 20 · Coralife BioCube 16 (LED) · Waterbox Peninsula 25 (AIO)
Sources: specs come from each maker's manual or product page, verified 2026-10-19: AutoAqua Smart ATO Duo, AutoAqua Smart ATO Micro, Hydor Smart Level, Tunze Osmolator nano 3152. The XP Aqua Duetto is left out because XP Aqua lists it as discontinued.