Gear
Auto top-off units: what reef keepers commonly use
Units that replace evaporated water with fresh water to keep salinity steady. Below are 2 items we see often, each with our honest note and where to view it in the UK and the US.
We sell nothing and earn nothing from these links. Prices are as each retailer's page showed them on the date checked, and they change. How this guide works
Listings checked . How we check
What should you look for in auto top-off units?
- An optical sensor plus a second guard: a maximum run time or a backup float.
- A siphon break on the delivery line so the reservoir cannot drain into the tank.
- A reservoir big enough for several days, sized with our calculator, but not so big that a fault could overdose fresh water.
Commonly used auto top-off units
Specs are as published on the listed pages. Where the US and UK pages give different figures, we show both.
Tunze Osmolator 3 Nano
Brand: Tunze
Our note. An optical-sensor top-off that keeps salinity steady in tanks up to about 250 litres. It is simple and well regarded; owners note the sensor cable is short and the pump fit in small containers is awkward.
Key specs
- Tanks up to 66 US gal (BRS) or about 250 litres (Tunze)
- Optical sensor, 0.5 mm accuracy
- 5 year warranty
UK listing
Amazon UK (UK): opens the retailer's page for Tunze Osmolator 3 Nano in the United Kingdom- Price as shown
- Price not shown on the page when we checked
- Checked
US listing
Bulk Reef Supply (US): opens the retailer's page for Tunze Osmolator 3 Nano in the United States- Price as shown
- US$149.99
- Checked
AutoAqua Smart ATO Micro
Brand: AutoAqua
Our note. A tiny optical top-off with its own pump and magnetic mount, sized for nano tanks and small sumps. It has a single sensor, and we found no current UK listing.
Key specs
- Pump 74 GPH, max head 6.5 ft
- 4.5 W at 12 V DC
- Optical sensor
UK listing
Not available in this region. We found no UK listing we could check on .
US listing
Bulk Reef Supply (US): opens the retailer's page for AutoAqua Smart ATO Micro in the United States- Price as shown
- Price not shown on the page when we checked
- Checked