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Our aquariums are a little taste of paradise. They give us the insider’s view into a world we can’t be a part of – I mean, unless you have gills. And a unique thing you may want to consider for yours is an auto top off system.

They’re soothing, interesting, beautiful, and often one of the things that help us find solitude and even peace in the middle of a chaotic world in which we stay busy all the time.

And for some of us hobbyists, our aquariums aren’t really a luxury. They’re a much-needed escape from a frenetic world that asks more and more of us each day.

So, when our aquariums ask more and more of us, too, it can be a little frustrating. Not impossible, but frustrating.

And that’s what the best aquarium automatic top off system helps with.

Best Auto Top Off System Quick-Find Table

Image Product

  • Uses a redundancy system for fail safe protection
  • Comes from 30 years of making high-quality auto topoff systems
  • Super easy installation with high-quality magnetic holders
  • Uses a redundancy system for fail safe protection
  • Comes from 30 years of making high-quality auto topoff systems
  • Super easy installation with high-quality magnetic holders

  • Simple, complete ATO system for an affordable price
  • Plug n play
  • Comes with optional siphon break
  • Simple, complete ATO system for an affordable price
  • Plug n play
  • Comes with optional siphon break

  • Small and compact
  • Incredibly easy to use
  • Uses recycled plastic bottles
  • Small and compact
  • Incredibly easy to use
  • Uses recycled plastic bottles

  • Quality brand
  • Compact design
  • Easy to install
  • Quality brand
  • Compact design
  • Easy to install

What is an Auto Top Off System?

auto top off system

I admit, when I first heard this term, I was super confused.

Auto top off… I vaguely guessed that it might be about refilling the tank when it needed it, but beyond that, I wasn’t sure what it could be or why anyone would pay money for such a thing.

And then, I did my research.

And experienced fishkeeping.

And saw what a difference this kind of device can make.

An auto top off system – or ATO – is a system that is connected to your aquarium and automatically detects when the water levels of your tank are getting low.

Then, using the reservoir tank of prepared water, it adds water back into the aquarium to top off your water levels.

Top tip: You can use them for either freshwater or saltwater aquariums, but they’re especially pertinent to the marine and reef aquariums because of the way a lower water level can effect the salinity and balance of the aquarium.

Why Would Anyone Use an ATO?

Now the question is, why in the world would anyone use such a device?

It might seem frivolous, and for someone who’s happy to constantly check the water levels and prepare the next round of water to add in, it is frivolous.

But for those of us who’d like to loosen up our schedules a little and worry less about the balances in our aquariums, they’re actually a very useful device.

You see, water evaporates every single day from your aquarium, whether it’s a cold water tank or a warm water tropical fish tank.

And that means that as the water evaporates, the salinity and other elements in your tank wind up out of whack.

And when your pH and other things get out of balance in the aquarium, your fish, live plants, invertebrates, and other critters can become unhealthy.

Author note: These systems do the work for you – automatically – so you don’t have to worry on a day to day basis. They also give you peace of mind when you leave town for a few days – or weeks.

You don’t have to teach your pet sitter how to create the right balance and how to measure and all of that.

When Does it Make Sense to Use an Auto Top Off System?

Beautiful aquarium with  fish in a room to modern apartment

You can use an auto top off system any time you’d like to reduce the daily tasks you need for dealing with your aquarium care.

They’re especially great for anyone who travels a lot, works late hours, et cetera, because you don’t have to worry about the water levels.

ATOs are also especially great for folks headed out on vacation. As mentioned above, they can reduce the stress levels both before leaving and while you’re away.

What Makes the Best Auto Top Off System?

There are several factors that can help you decide which ATO is best for your aquarium and lifestyle. Take a few minutes to at least skim the sections so you know what to look for and how to decide.

What Comes with the Kit?


The first thing many folks consider when choosing the right auto topoff system for their aquariums is what comes with the kit.

Most kits will include a self-leveling controller, one or two float sensors, and the mounting implements needed to properly place the ATO. The system may also include pumps and hoses.

So, if you want to customize, you’re probably looking for the basic kit, but if you want it all-in-one, be sure to find a system that has the hoses and pump included.

What Does it Take to Install It?

For seasoned pros, this isn’t necessarily as much of an issue, as you probably already know what you’re doing.

But for anyone who’s not been doing fishkeeping for a long time or has never used an ATO before, the installation requirements are probably something worth considering.

For those of us who aren’t that handy, it can be a real hassle if we choose the wrong aquarium ATO.

Top tip: Make sure the reviews – both by pros and newbies – indicate the system is easy to install. Specifically, look for magnetic mounts. These are the easiest to install with.

What Size Aquarium Do You Have?

Next, you need to consider the size aquarium you own. The auto top off systems come with different size pumps that work effectively in specific gallon tank sizes.

Author note: If the pump is too weak, it will take too long to refill the aquarium, which is kind of the whole point.

What’s Your Budget?

Finally, you need to keep your budget in mind.

Fish tank auto top off systems are important, for sure, but your budget needs to have the proper allowance to get a decent one.

If you’re not in a huge hurry – i.e., you don’t have a lengthy vacation coming up – and your budget is small, it’s best to wait until you’ve got a little more accounted for the ATO.

Author note: It’s always best to get a higher-quality option a few weeks from now rather than a piece of crap today.

Reviews of the Best Automatic Top Off Systems Available

To find you the best suggestions possible, we dug through tons of reviews and information, spent hours studying and learning, and made note of the absolute best auto top off system options we could find.

We determined these great options by looking at:

  • Various sizes to suit all needs
  • Varying budgets to ensure at least some are high-quality and budget-friendly
  • Solid manufacturers that we trust in the fishkeeping world
  • Easy installation options
  • Varying levels of “kit” available to meet everyone’s needs

#1. Tunze USA


The Tunze Osmolator Automatic Top Off is easily the best auto top off system aquarium setup we’ve seen.

We saw many other highly rated ones on multiple sites – like the JBJ Automatic Top Off System – but the others just didn’t beat this one.

It has a redundancy system for safety, is easy to install, and comes from 30-years’ experience by Tunze, making high-quality auto topoff systems, meaning they know their stuff.

  • Dimensions: 4 x 1 x 2.5 inches
  • Installation type: Easy magnetic installation
  • Bonus features: Redundancy for safety

What We LoveD

The very first thing we have to talk about is the reliability of the system.

The Tunze auto top off reef tank system is great at going, going, going. It’s extremely reliable at doing its job, and that’s one of the most important things in such a critical system.

We also love the redundancy system that adds an increased level of safety to the whole thing. Always better to be over-prepared than under!

We love the long-lasting reputation of the Tunze company. While brand name certainly isn’t everything by a long shot, it sure does make a big difference in giving us assurance we’re going to get a high-quality device that will last.

We also, of course, love how easy this auto topoff system is to install. The magnetic holders make it a snap. And since the whole point of the system is to make life easier, well, we just love that even installation does.

The whole system is small in size, which is fantastic. The whole point of an aquarium at home is to add to that calming, de-stressing atmosphere, and unsightly, ugly equipment can take away from that.

But since this system is so small, it doesn’t mess with aesthetics or space.

Finally, and perhaps most importantly, the Tunze automatic top off system is really precise at estimating the water levels. Since, again, this is the point of an ATO, that makes this an extremely valuable little piece of machinery.

What We DIDn’t Love

The only thing we don’t love about this ATO is the price tag. It’s not ridiculous by any stretch, but it would be nicer if it were a little less expensive.

Even so, it’s well worth the investment.

Our Verdict on the Tunze USA Automatic Top Off Osmolator for Aquariums

Based on all our research, study, personal review, and understanding of auto topoff systems, this is hands-down the best ATO you’ll find on the market.

It comes from a long-line of predecessors – they’ve been making these systems since the 1980s! – and are made of high-quality materials.

They’re easy to install, easy to maintain, and even though they’re a little pricey, they’re definitely worth the investment for the assurance your aquarium will be safe when you’re away.

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Digiten ATO

For anyone on a tight budget, the Digiten ATO system is a great choice for your auto top off system.

The kit comes ready to go as a “plug ‘n play” – meaning it’s just plug it in and you’re ready to hit “start.”

The system is complete with everything you need and works with a nice relay control for precise water level with overfilling protection

  • Dimensions: unknown (but moderately small)
  • Installation type: Not specified
  • Bonus features: Comes complete with everything you need for the system – and no moving parts that can break

What We Loved

We love how simple the kit is and how easy it is to install and use, no matter your experience or skill level.

The system uses a relay control with a precise dual float water level sensor that prevents overfilling – and actives when the water goes below the minimum level for your setup.

There’s an optional siphon break installation part available, too.

We love how versatile this ATO system is – suitable for a wide range of systems, all at a budget-friendly price.

Ultimately, we love that the system is a plug ‘n play, meaning you set it up directly from the package without special installation instructions.

What We Didn’t Love

The system is definitely not designed for larger aquariums. Make sure you only use this on fish tanks smaller than say 35 gallons.

Our Verdict on the Digiten ATO System

Overall, the Digiten ATO system is a great choice for folks on a tighter budget looking for a quality ATO system.

The Digiten is made well with quality materials, and provides your basic needs in an ATO system – as long as your aquarium isn’t larger than the system is designed for.

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Skemix auto top off system

For a super simple, small fish tank ATO to leave “in charge” while you’re away, the SKEMIX system is a great, budget-friendly choice.

One unique thing about this system is that it uses recycled plastic bottles.

All you have to do is screw the compensator to the water bottle and attach to your tank. Voila! Ready to go.

  • Dimensions: unspecified, but pretty tiny!
  • Installation type: direct attachment (slip on)
  • Bonus features: Works with recycled water bottles

What We Loved

We love the float valve, simple construction, and lack of wiring or power required for this tiny tank ATO system.

The simple, easy system is a near instant install and anyone can use it. Simply attach a water bottle and then place on the tank. Voila. You’re done!

The system comes with 3 slots to interchange, making it work for four different thickness of tank walls, meaning you should be able to fit it onto any small tank you’ve got.

What We Didn’t Love

The only thing we don’t really love about this is that it really only works on small aquariums. We love the simplistic, no-electric design, so it’s a little sad it only works for small tanks.

Our Verdict on the SKEMIX ATO System

The SKEMIX ATO system is a super simple, easy to install, easy to use, easy to care for option for small tanks. It does the trick and does it well.

The system requires no electric power and no special equipment. You just install a recycled plastic bottle and attach it to the edge of your tank.

The only potential issue is that it is definitely limited to small tanks.

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The Innovative Marine Hydro Fill TI ATO Controller is a great little ATO system for use in either freshwater or saltwater aquariums.

The dual sensors that come with the system are fairly reliable, easy to interpret, and the whole system is pretty easy to set up.

Plus, it gives you the ability to customize your own high and low set points for prevention of that constant on/off cycling that single sensor systems tend to have.

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How to Properly Set Up Your ATO


There are several steps you’ll need to take to set up your ATO. Of course, each system is a little different, and you should always verify the specific directions yours comes with, but most of them are fairly similar.

And just to give you an idea of what you’re getting into, we’ve got those basic steps right here.

  1. Gather your supplies together. These will come together in your kit or include the pump and tubes that you purchased separately.
  2. Power on the system to test everything – the sensors should light up and the pump should work.
  3. Prepare the empty reservoir. Don’t fill it up yet but get it ready. It’s easier to work with empty but cleaned with purified water.
  4. Prepare the sensors by placing one in the reservoir and the other in the sump just above the water line.
  5. Install the pump. There should be two outlets for the pump, one labeled in and one labeled out. Run the tubing from the reservoir to the inlet. Run the other tube from the outlet. Try to keep the tubing above the water line while installing. If it goes underwater, it can accidentally create a back siphon that will suck water from the fish tank into the reservoir.
  6. Next, plug in the device.
  7. Finally, fill up the reservoir and watch your auto top off system do its thing.

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