Auto Top Offs (ATO) Biggest Secret

Running an Auto Top Off (ATO) is one of the easiest ways to keep your tank running smoothly. An ATO unit keeps a constant water level in your tank, which makes sure your protein skimmer runs correctly and keeps your salnity in check. The result: happier, healthier tank.

One of the common mistakes I see people make with ATO’s is that they spend WAY too much money on them. The biggest secret of ATOs is that you can get one rolling really cheap – so long as you aren’t a reef junkie.

For tank dabblers and reef enthuiasts, I recommend the dual switch unit. Cost: $50 plus the pump. The Tom Aquatics Aqua-Lifter pump works great for the water pump part of this ATO. Total cost for this setup: $65.

Yes, there is nothing sexy about the unit, but seriously, it is an ATO we are talking about here. It is going to sit in your sump, or in the back of your tank – you’re never going to see it. As long as it works (mine has worked great for the year I’ve had it), who cares what it looks like?

Now, if you are a reef junkie, you’ll want to opt for the Tunze Osmolator 3155 ($213) because you’ll be dazzled by the fact that there is no float switch involved, only a photovoltaic sensor. It also includes a float switch as a backup sensor in that if the osmolator pumps too much water into your tank, then the float switch will rise and turn off the pump. Also, the osmolator won’t run for more than 15 minutes which adds another layer of protection.

As with most things in the saltwater tank world, you get what you pay for.

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Comments for this article (7)

  • Luis says:

    Hi there,

    I got Tunze Silphos 920 would you recomended to use it like granular ferric oxide ?

    Thanks for the help

  • Bryan says:


    Just curious if you somehow connected this system to your Apex. I was thinking about a snail or aquarium gunk bypassing my precautionary methods and keeping float sensors from working right. Just a “what if” my wife called me with a problem and having the ability to remotely pump or stop pumping was needed regardless of the sensors.

    The reef-techno person in me was wondering the different ways this could possibly be wired to the Apex, other than just plugging the doser in the strip. Can float switches be wired to the controller? Maybe I should stifle the question until I get one and get familiar with the software and controller. ha Man I need to go to bed.

  • Bryan…you can connect it to the Apex, no problem. I’m doing it right now for an APEX system I’m building for someone.

  • Bryan…float switches can be wired to the APEX. I just set one up tonight that way. Do you already own the APEX or are you considering it?

  • The Berrier's Reef says:

    Got the same A.T.O. as Mark, works great.

  • Jesse says:

    Hi Mark,

    Thank you for your website videos and comments. I am new to the scene and your posts have been very helpful. I have a question about the ATO “reservoir”. I am making RO/DI water into a 32 gallon container in my basement on the other side of my house. It would be “possible” to run a water line through the basement ceiling up the wall behind my tank and into my sump (for my 90 gallon aquarium), but I was wondering how much water I really need in the reservoir. Maybe I can put a small container next to my sump beneath my tank that hold new RO/DI water for the top-off. Or would this defeat the whole point of having an auto top off system? The RO/DI water holding tank would require about 40′ of pipe to my sump and it is up one floor. If you do suggest getting the water from my RO/DI holding tank, what pump do you think would work well in this scenario and how big should the water line be?

    I would consider myself a “reef enthusiast”.

    Thank you,


  • joe says:

    I got the auto top off and I run the apex lite and I was wondering how to hook up the auto top off to my apex could you please post how you hooked up yours. Thanks Mark

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