ATO Systems: Solving Evaporation and Salinity Swings in Reef Tanks

Every reef tank loses pure water to the air constantly. Salt does not evaporate, only water does, so as the level drops the salinity of what remains slowly climbs. Left unchecked for even a day or two in a warm Brisbane room, that drift can push specific gravity up enough to stress coral tissue and throw off dosing calculations that assume a stable volume. An ATO exists to solve exactly this one problem: replace evaporated freshwater automatically, before it becomes a swing.

Manual top-off works fine for some keepers, but it means topping up at the same time every day without exception. Miss a hot weekend away and salinity can jump noticeably by the time you get back. An ATO removes that dependency on human memory and turns top-off into a background task the tank handles itself.

How the sensing works

There are two common ways an ATO decides when to add water. Float switch units, like the AUTOAQUA SMART ATO, use a small float that rises and falls with the sump water level. When it drops below a set point the float triggers a pump to run until the level is restored. They are simple, reliable, and easy to service, though the float itself needs an occasional wipe to stop biofilm from making it stick.

Optical sensor units take a different approach. The AUTOAQUA SMART ASOV uses an infrared optical sensor with no moving parts in the water. Light either passes through water covering the sensor tip or hits air, and the unit reads that difference to decide whether to run the pump. Because there is nothing to physically stick or jam, optical sensors tend to be less fussy about biofilm and algae buildup over long runs, which matters in a humid Queensland fish room where growth on submerged plastic happens fast.

Fail safes worth understanding

Any ATO can, in theory, run away if a sensor fails wet or a pump sticks on. Good units include a maximum run timer that shuts the pump off after a set number of seconds even if the sensor still reads low, preventing a freshwater overflow event if something goes wrong overnight. When choosing a unit, check that this safety timer is adjustable so it suits your sump volume and pump flow rate rather than a generic factory default.

Reservoir sizing for Brisbane conditions

Evaporation rate depends on tank surface area, ambient humidity, glass lid coverage, and how hard your return pump agitates the surface. A 400 litre reef system in an air conditioned Brisbane living room might lose one to two litres a day. The same tank in a garage with open sump and no lid, through a hot October week, can lose considerably more. Size your top-off reservoir for at least three to five days of buffer so a missed refill does not leave the pump running dry and cycling on air, which shortens pump life and can trip alarms unnecessarily on systems tied into a controller.

Speaking of controllers, both float switch and optical ATOs can usually be wired into a broader monitoring setup so a failed top-off or a stuck float triggers a phone alert rather than a surprise the next morning. If you are already running or considering a controller for dosing and temperature, check the controller range for units that integrate top-off monitoring alongside other parameters.

Installation notes

Mount the float or optical sensor in the sump return chamber, away from turbulent bubble zones that can cause false triggers. Keep the top-off pump outlet aimed so fresh water disperses rather than blasting directly onto the sensor. Use rigid or semi-rigid tubing where possible to reduce kinking, and check fittings against your existing plumbing sizes before ordering, since barb diameters vary between brands. Browse plumbing fittings if you need adapters to marry a new ATO pump line into an existing bulkhead or check valve setup.

ATO units see a fair bit of turnover through our range as Brisbane's warmer months push evaporation demand up and keepers look to automate before they travel over summer, so check out our stock list to see which float switch and optical models are currently moving through, since exact configurations shift with each restock.