Your reef tank is going to evaporate water, something you can see with the naked eye as the water level drops. What you don’t see is as the water leaves the tank, the salt and other dissolved elements stay behind. That causes the water in your tank to become more concentrated and salinity levels will continue to rise until you replenish the tank with freshwater.


Manual Top Off


Manual Freshwater Top Off

This is the part most new reefers get wrong day one. For some reason, it seems like you should top off a saltwater tank with salt water but that’s not the case. If you do that, you are going to increase the salinity every time you add water.


When water evaporates it is only freshwater leaving the tank, all of the salt stays behind. So when you replace the evaporated water, commonly referred to as topping off, you need to use fresh water, ideally purified RO/DI water.


40 Gallon Tanks


A 40-gallon tank will evaporate about ½ gallon per day, give or take. That means you need about 3.5 gallons of purified freshwater per week. With forgiving corals like the ones we selected, you can probably get away with topping it off once per week but you really want to do it daily. The more often the better for stability’s sake.


ATO Storage


Just keep a 5-gallon bucket of fresh RO/DI water under your tank or close by and simply scoop out water to replenish the tank when you do your daily feeding. It really only takes about 30 seconds and will ensure stable salinity levels in your reef tank. That is all there is too it, manually add freshwater daily to keep your salinity and tank water level stable.


Tunze Osmolator 3155


Automatic Top Off Systems (ATO)

An auto top off system (ATO) can reduce the work involved with topping the tank off by doing it for you automatically, which in turn increases tank stability. Stabile salinity means a majority of other elements in the water are stable as well, at least on a day to day basis.


An auto top off system contains a water level sensor that senses or recognizes the water level in your tank. When the water level drops with evaporation, that sensor will trigger a water pump to then deliver freshwater into your aquarium. When the water rises back to the appropriate level, the sensor will then trigger the pump to turn off.


Tunze Osmolator 3155


The Tunze Osmolator 3155 is, by far, the most popular and reliable ATO system on the market. Trust is important with an ATO system because if it gets stuck ON, they can flood the tank and your floor with water. Anyone who has used a low-quality ATO and had it flood will tell you the same, don’t choose to learn the hard way!


Tunze ATO water level sensor


The Tunze ATO relies on a combination of two sensors, an optical eye that monitors the precise water level and triggers your pump which is then backed up with a mechanical float switch to shut down the system in the event of an overfill. It also has an internal maximum fill timer and adjustable pump speed. All of this works together to ensure the unit never fails in the ON position and pretty much exactly why it is the most trusted unit on the market.


An ATO is one of those specific pieces of gear that almost everyone ends up with so you either get one now or just manually top off until it fits your budget. There really is no need to contemplate whether or not you need it because you do.


Tunze Nano Osmolator


If you want to save a few dollars, Tunze does make a Nano Osmolator which is available for half the cost and suitable for tanks up to 50 gallons. The big difference is the lack of an optical sensor on the Nano unit. It relies solely on a mechanical float switch and internal timer to prevent overfill.


With our 40 gallon tanks here, we are going to use one of each Osmolator. The 40-gallon breeder will use the Tunze Nano Osmolator with a simple 5-gallon bucket to store freshwater.


Red Sea E170 ATO system with Trigger Systems Reservoir


The Red Sea E170 will get the traditional Tunze Osmolator 3155 paired with a Trigger Systems 10 Gallon ATO reservoir.


By now, I am sure you are catching onto the theme and why we chose to build both of these tanks. Both tanks will produce the same level of success so it is not important which route you take in terms of success. Simply choose the tank that best suits your budget and overall desired form factor.


What’s Next

You can now check evaporation off the list and at this point, the tank should be operating smoothly from day to day. Next up is learning how to keep things looking sharp and the first step is cleaning your tank’s glass.


Just like almost all of these topics, there are a dozen ways to get this done but with the right advice, you can choose the best tool for the job that suits your desires.


5 Minute Saltwater Aquarium Guide Episode #21


Looking for a different topic or have questions? You can binge the entire 5 Minute Saltwater Aquarium Guide playlist right here on our website. We also invite you to join the #askBRStv Facebook Group which is a free resource for you to ask questions, get advice, interact with other hobbyists and get your daily reef aquarium fix.