Various causes of evaporation in aquariums

An Auto Top Off is one of the most essential pieces of equipment for any reefkeeper. Here’s why.

The world's oceans are very stable. Stable
temperature, stable parameters, and stable salinity, and corals have evolved
over millions of years to only thrive in a very narrow set of water parameters.
Oceans evaporate, water molecules form clouds, then rain, then rivers return
all that freshwater to the sea, keeping the saltiness of the sea in
equilibrium. And our saltwater aquariums are no different.

In tanks, water evaporates, removing the
freshwater part, but leaving behind the salt. As the water level drops through
evaporation the water becomes saltier and saltier, eventually becoming too
salty even for marine life, and livestock will start to suffer. Leave your
saltwater tank running without ever topping it up and eventually all that water
will evaporate, leaving a thick brine and eventually a totally dry, salt
crystal-covered tank. Return pumps and wavemakers start to suck in air as the
water level drops, causing millions of tiny bubbles in the water, and making
evaporation even worse. Pumps eventually run dry, risking overheating, seizure,
the ecosystem fails, and everything dies. But all this can be avoided…

Auto top-offs (ATO) are an invaluable piece of equipment for all marine aquariums as they top-off evaporation and keep tank salt levels stable, something which is essential for corals and invertebrates. By keeping the water levels constant in sumps, ATOs also prevent return pumps from sucking in air and running dry, keeping system water circulating from tank to sump, and then back again.

How Auto Top Offs Work

Pump, Sensor, and Controller of XP Aqua Duetto

An ATO consists of a sensor, a pump and a controller. The sensor is placed on the water line of the pump chamber, and the pump is placed in a separate tank or drum of freshwater, Usually RO water, DI water, or water which has been prefiltered both ways.

When evaporation occurs the water in the pump chamber will start to drop, exposing the sensor to the air. The sensor sends a message to the control box that it isn’t touching the water line anymore and the controller turns on the pump in the ATO chamber. Water is pumped from the ATO reservoir to the pump chamber until the sensor is once again touching the water. When this happens the sensor tells the controller, and the controller turns off the ATO pump. An ATO will run night and day, 365 days a year, constantly replenishing water lost through evaporation, and keep your salinity as constant as the day you first filled the tank. Auto Top Offs are often the first piece of automation deployed on any reef tank and they literally save lives. They’re brilliant.

ATOs aren’t just for sumped systems however and if your tank has a filter in the back, or even no filter section at all, just place the sensor at your required surface water level in the main tank and it will do the same, very beneficial role, as one placed into a sump.

What you need to know about Auto Top Offs

ATOs can vary in price and quality, but our advice is not to cut costs on something so essential. Quality and accuracy of the sensor vary, as do the ATO pumps and controllers. It's not unheard of for a sensor to be high and dry while the controller fails to top-off the evaporation.

A good ATO will also have another level sensor above the first one to set off an alarm to shut off a pump that continues to overfill a pump chamber - offering extra peace of mind and stability. Level controllers are also available separately to shut off any pump if water levels get too high in sumps or protein skimmer collection cup. Monitoring systems that will send you an alert if water levels get too high or too low due to ATO failures are also available. You will need to top off the top off reservoir yourself on a regular basis, with evaporation in excess of seven gallons per week not being out of the ordinary.


When you set up your ATO device ensure that the hose from the top-off pump doesn’t terminate too low, as if it does, when the pump is switched off by the controller, water can continue to siphon into the system. Make sure that the hose is also securely fastened too, as ATO pumps may be small but they are high pressure, and if that hose comes loose it will pump water onto your floor until the ATO reservoir runs out of water.

You also need to turn your ATO off when conducting a water change, because when you remove water you are removing salt too, so the level change will cause the ATO to pump freshwater into the system before you replace the water removed with saltwater, and salinity will change. And if you remove something which displaces a lot of water, like a rock, or a protein skimmer, the ATO will immediately kick-in, so turn it off.

Sensors with cheap rubber suckers may not be
as reliable as those sensors held on to sump glass with magnets, and
occasionally an ATO which has itself previously ran dry, may not instantly pump
water as it should. Narrow top-off hoses can block too.   

Other types of Auto Top Offs

Float valve on the Red Sea Reefer Auto Top Off

Some aquariums like the Red Sea Reefer already come with an integrated top-off system. There, the Auto Top Off reservoir sits above the sump while a simple float valve sits in the return pump chamber. Water evaporates, causing the water level in the pump chamber to drop. The float valve drops with the water level, opening the valve, and releasing top off water from the reservoir above, via a plastic tube, and gravity. If you purchase and run a system like that, you won’t need a separate ATO device, although a DIYer may prefer the greater controllability from an electronic ATO, and choose to modify their system and upgrade.

Some Auto Top Offs can also be used in conjunction with a solenoid valve and RO system, to pump freshly made RO into the return pump chamber and avoid the need for a separate ATO chamber, or you having to fill that chamber manually.