Saltwater aquarium or reef tank is a high-maintenance hobby. From feeding the fish to cleaning the aquarium – the list of daily chores adds up. Add in the fact that you have to replace the water that evaporates each day to maintain healthy salt levels and avoid your fish tank getting too dry, and you may find yourself overwhelmed.

Thankfully, a lot of those chores can also be automated using various pieces of aquarium system technology.

In this article, we’ll be discussing about the best auto top off systems; instruments that make sure your aquarium never runs out of water. From why you need them to where to find the best one, we’ll be covering everything you need to know on automating water top offs in your aquarium!

Before going into the details let’s take a look at our top picks for auto top off systems –

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Reviews of 5 Best Auto Top Off Systems

1- Tunze Osmolator 3155 Automatic Top Off for Aquariums

Compact and reliable, Tunze osmolator is a system that uses two different sensors to measure water levels. Its optical sensor functions as a primary sensor, while its float sensor is a back up that prevents potential malfunctions. It also comes with a pump and all the tubing necessary to set it up right away. And as an added bonus, it is easy to set up and comes with very clear instructions.


  • two sensors
  • great technology and materials
  • very compact
  • easy to install


  • more expensive than others on the list
Tunze 3155.000 Osmolator Auto Top Off System
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2- Hydor Smart Level Controller Automatic Top Off

Built with quality materials, the Hydor Smart Level ATO Controller is meant to be used in either freshwater or saltwater aquariums, and is rust-resistant. One of the things that we love about this device is that it has an alarm system that lets you know through sound when the water level has fallen low, or if there is any malfunctioning with the mechanism. It also comes with a pump, and is very energy-efficient. So if you don’t mind sound alarms, Hydor Smart Level Controller is a very effective system that has you covered with everything you need to set up your new automatic top off system.


  • sound alert
  • energy efficient
  • good quality materials
  • reliable sensor


  • the sound alert cannot be turned off and can get annoying overtime
  • lower quality pump
Hydor Smart Level Controller
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3- AutoAqua Smart ATO

This ATO system by AutoAqua comes with an optical sensor and a pump. With both a reliable sensor and a reliable pump, it is a great package that gives you everything you need to get started. The only thing to be mindful of is that optical sensors need a certain level of cleanliness to be able to measure the water level. If this isn’t an issue, this is an auto top off system we’d highly recommend!


  • everything included in the packaging
  • reliable technology


  • optical sensor needs clean tank
AutoAqua Smart ATO Lite
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4- JBJ Automatic Top Off Water Level Controller

Coming with two float sensors, this is a good, affordable auto top off device. It comes with a clever LED display system that lets you check water levels at a glance, and is easy to install. On the other hand, this device doesn’t include a pump, and has been known to let the pump run dry. It also emits sounds when the pump is activated, which some users may find to be annoying.


  • dual sensor
  • LED display
  • sound alerts


  • no pump included
  • makes little beeping sounds
JBJ Automatic Top Off Water Level Controller for Aquarium
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5- Ultralife Reef ProduCounts AULFS10A Deluxe Mechanical Float Switch

Affordable and reliable, this float switch by Ultralife is a great device for those with reef aquariums. This one comes with only the very basics: a float switch made from good quality, resistant materials. You’ll have to set up the pump yourself, which also lets you choose what type of pump you want within your budget. Overall, it is a very simple option that you’ll have to program and install yourself, but it is solid and reliable.


  • good materials
  • affordable


  • float switches can sometimes malfunction
  • no pump included

Honorable Mention: XP Aqua Sumpless ATO – All in One System for Sumpless Aquariums


If you don’t have a sumo installed in your reef tank then this XP Aqua Sumpless ATO is just the right thing for you to keep the salinity levels correct. On top of that it works as a surface skimmer and can host filtration media in the chamber. Cool, right?

What is an auto top off system?

An ATO system is a device which measures how much water has evaporated from your aquarium, and replaces it to make sure that the water level is maintained consistently.

There are three main components to an auto top off system: a sensor, a water reservoir, and a pump.

The sensor is what the ATO system uses to determine how much water is left in your aquarium, and whether the level needs to be readjusted.

The water reservoir is what you’ll be pouring the water for top-offs into.

Finally, the pump is what gets the water from the reservoir back into the aquarium.

Although all ATO systems contain these three components, there are some significant variations. In the second part of this article, we’ll be showing you five great options for ATO systems, and what makes them unique.

Why having an ATO system is important

But first, let’s discuss why having an auto top off system is so important. The main reason why you should consider investing in an auto top-off system is that low levels of water are genuinely dangerous for your aquarium inhabitants.

In a saltwater aquarium, the salinity level needs to be maintained at a healthy level. That is to say, your saltwater aquarium can’t have too much salt, or not enough.

The problem is that, as water naturally evaporates from your tank, the salt stays in. With time, the concentration of salt increases, which can be very harmful to the health not only of your saltwater fish, but also of reef and plants.

The solution? Well, you can either monitor your aquarium closely, and replace the water that has evaporated manually everyday.

This isn’t a big chore, but it can get tiring, especially if you have a larger tank from which a lot of water evaporates each day.

The alternative is to get an ATO system that takes care of this daily task for you.

As an added bonus, having an ATO system means that you can leave town for a couple of days without having to worry for your saltwater aquarium.

Best Auto Top Off System For Reef Tank

How to choose the best auto top off system for your aquarium or reef tank

So you’ve decided that an auto top off system would make your life a lot easier. Good. But with so many options on the market, how do you find one that really works for you? There are a few different factors to take into consideration: sensors, reservoir size, the pump, extras, and of course, your budget. Let’s have a closer look at them:

The Sensor

The sensor is the most important part of the ATO system; it’s what determines whether the water levels are normal, or if they need to be topped up. There are quite a few different types of sensors out there, from an optical sensor to a temperature sensor. As a general rule, more expensive sensors tend to be more reliable, while cheaper ones can malfunction easily. Let’s see what they each do:

Float Switch

A float switch is a magnet-based sensor that determines tank water levels. When the water level drops too low, it activates a circuit, leading the pump to add in water to the tank. Simple and effective, this particular system can malfunction easily, especially if the surface of the aquarium gets dirty because of mineral buildup.

Optical Sensor

An optical sensor determines tank water level by shining an LED light through the water. If the light receiver detects that the ray didn’t go through water, this means that water levels are too low. The system is then activated and more water is topped up in the aquarium.

Temperature Sensor

Temperature sensors tend to be very reliable. They work by giving a signal when they detect a temperature change. When those sensors are no longer submerged in water because it has evaporated, they detect a higher temperature, and give the pump a signal to top some water up.

The Reservoir

As a general rule, aquarium ATO systems don’t come with their own reservoirs. You’ll have to make your own out of a jug, another fish tank, or even a plastic bin. Whatever you use, you need to make sure that your reservoir is large enough to top off your tank a few times. You would be amazed at the amount of water than can evaporate from an aquarium! So assess the tank size and its water needs, and you’ll be able to find the reservoir that works best for your new auto top off system.

The Pump

The water pump is what transfers the water from your reservoir into your tank. It is vital to get a reliable pump because it will be in use daily. Be aware though, the ATO system that you purchase may not come with a pump already included. If it doesn’t, it will come with a female plug that can be plugged into a pump easily. It is then up to you to choose which pump you want to buy. We recommend a dosing pump because they tend to be more reliable, are less prone to failure, and don’t risk making your tank overflow.

What Should My Auto Top Off System Contain?

When choosing an automatic top off system, don’t just go for the cheapest one; see what’s included in the package. Some will include a pump while some won’t. All of them will contain a sensor, though it may differ. And some also contain a few extras, like a malfunction alarm. So pay close attention to what’s included in your automatic top off system, and choose the one that offers you the best value for money.

Your Budget

Finally, your final consideration should be your own budget. An automatic top off system is a long term investment, so if you have money to spare, we would recommend purchasing a very reliable one. This will ensure you never have to fear for your aquarium getting flooded, or the system malfunctioning. However, you will also be able to find very solid machines for an affordable price. Read on to find our best picks!

How to set up an auto top off system?

So, once you have your new ATO system, what do you do? The process will depend a bit depending on whether the package that you chose came with a pump or not. If not, make sure you get a good, compatible pump and proceed. You will also need to find a reservoir for the water.

Once you have all the necessary equipment, read the instruction manual of your ATO system carefully. It will provide all the detail on attaching parts, testing out the device, and safety measures.

Begin by installing your reservoir in a safe place where it doesn’t risk being knocked off, and install the tubing so that the pump reaches it. Next, connect the pump with the sensor of your ATO system. We would recommend testing the device first by removing water from the tank of the aquarium and checking if the sensor and pump are in working order.

Finally, be aware that sensors should be tested a few times a year, to ensure that they are working properly. The danger with a malfunctioning sensor is that it may not give the pump the correct information, causing your aquarium to lose too much water and see its salinity level increase or worse, overflow.

Final Words

When it comes to keeping the water levels of your aquarium consistent, auto top off systems are a god-sent. Easy to install and set up, they are the perfect solution for keeping your aquariums from going dry as a result of evaporation without you having to constantly transport water into the tank.

And fortunately, there are many great options to choose from for every budget. We hope that this guide has been helpful to you, and showed you what auto top off systems can do for you. After all, they are one of the best pieces of aquarium technology to save you some time—and keep your fish happy!

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