The Best Automatic Top-Off Systems for Your Reef Tank in 2023

When it comes to owning a saltwater aquarium, maintaining a constant salinity level is vital to the health of your fish and corals.

Getting the best automatic top-off system will reduce daily maintenance and keep water chemistry stable.

In this guide, I’ll review five ATO systems and discuss how to select the right one for you.

ATO Systems Overview

Overall, the Tunze’s Osmolator Universal 3155 is the best ATO system.

Last update on 2023-09-20 / Commissions Earned / Images from Amazon Product Advertising API

The Best Automatic Top-Off Systems Reviewed

1. AutoAqua’s Smart ATO

Including everything you need to get started, it’s the unique design of this model that catches most hobbyists attention.

The sensor and controller are built into one module which attaches to your aquarium using a magnet.

It works by sensing the water in your tank through your aquarium or sump walls, so it’s only suitable if you have a clear tank or sump (wall thickness should not be more than half an inch).

This ATO system features an optical sensor, so no moving parts, eliminating the issues we previously discussed about floating sensors.

You must keep the sensor clean though. Or it won’t be able to accurately detect the water level.

And if you’re put off by the fact it doesn’t have a backup sensor, don’t worry. It uses a smart internal program (AFS Technology) to cut the power to your pump based on how many times, and how long the pump cycles in a given period of time.

So you’re still protected against over-fill and dry-runs.

The pump included has a GPH of 74, is low-voltage, safe, comes with 6 feet of tubing, and can deliver water safely to a max height of 6.5 feet.

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2. JBJ’s ATO System

The JBJ ATO system accepts any 110/120 VAC pump with a maximum capacity of 2 amps.

With a low price tag and dual float switches, the JBJ’s ATO offers you great value and is reliable enough to get the job done if you’re on a tight budget.

The kit doesn’t include a pump, but, it’ll work perfectly with any 110/120 VAC pump with a maximum capacity of 2 amp.

This unit has a maximum fill time of 14 minutes, so if you have a larger tank, you’ll probably want to go with a submersible pump over the slow dosing pumps.

You can also adjust the fill time, you can dial this to suit your tank size and further protect yourself against over-fills and dry-runs.

What I like about this ATO system is the indicator lights. They make it so easy for you  to know the status of your sensors and pumps.

The dual float switches use suction cup mounts for placement and are replaceable. Personally, I don’t like suction cups. I find they become loose, causing the ATO system to operate incorrectly.

And because the JBJ’s water level controller relies on the floating sensors, both must always be working perfectly. You’re going to have to make sure you keep them clean.

Last update on 2023-09-20 / Commissions Earned / Images from Amazon Product Advertising API

3. Tunze’s Osmolator Universal 3155

The most popular and reliable automatic top-off system available at an affordable price.

Tunze’s Osmolator comes with everything you need to get started and offers outstanding protection against over-fill. It’s really hard to beat.

The pump included is a submersible, low-voltage metering pump with a maximum head pressure of 7.2 feet and comes with 6 feet of tubing.

Featuring an electronic optical water level sensor, waves and water moment won’t affect the measurements.

If for any reasons your water rises above this sensor, your mechanical floating switch will cut the power to your pump.

Even the pump is smart–automatically turning off if it runs for more than 10 minutes (further reducing your risk of over-fill and dry-runs), and has a minimum run time of 20 seconds to prevent pump cycling.

The kit includes strong magnetic sensor mounts (no suction cups!), making it easy to install in any aquarium or sump with wall thickness up to ¾ of an inch.

You can actually do multiple mounting configuration by using one or both of the magnetic sensor holders. And the kit even includes some strong Velcro so you attach the controller to the inside of your tank stand.

Multiple LED indicator lights will clearly display the status of your sensors and pump at all times. But remember, the sensor is optical, so you’ll need to make sure it’s clean.

Last update on 2023-09-20 / Commissions Earned / Images from Amazon Product Advertising API

4. Hydor’s Smart Level ATO

Featuring a unique three-pronged temperature sensor to monitor your water level, it eliminates a lot of issues seen with ATO systems like calcium, algae, and water movement.

It doesn’t include a pump, but the Hydor Smart Level ATO system accepts 110/120 VAC, two pronged pumps with a maximum capacity of 50 watts.

You get a high-level of protection for a budget-friendly price.

The audible alarm will sound if the third high water level sensor is triggered–cutting off power to your pump. Status LEDs are included as well as a magnetic sensor mount, so again, no annoying suction cups.

You just need to make sure you keep the sensor clean.

With a maximum fill time of 10 minutes, you’ll need a pump with the ability to deliver all the water you need within that time frame. So if you have a larger tank, a submersible pump might be your best option.

And keep in mind, Hydor’s Smart Level has a minimum run time of 25 seconds. So choose you pump carefully. Especially if you own a nano reef aquarium.

Last update on 2023-09-20 / Commissions Earned / Images from Amazon Product Advertising API

5. MagTool Optical Sensor Auto Top Off with Upgraded QST Tech and Smart ETFT

This top off system is unique because of its smart tech. 

It has an optical sensor, but that’s only the beginning. 

Included is the extra time filling technology (ETFT) and the anti-fluctuation technology (AFT).  

These systems learn how long it takes to refill your tank on average. 

The system will automatically stop the pump and give a visual alert if it senses that the water has been flowing longer than should be necessary to fill your tank.

This should help prevent dry runs and overfills.

Everything you need to get started is included with the kit; it comes with a 70 GPH pump. 

The sensor mounts on the tank wall (up to 15 mm thick) using strong magnets.

Last update on 2023-09-20 / Commissions Earned / Images from Amazon Product Advertising API

Which ATO is Top-Off The List?

It’s Tunze’s Osmolator Universal 3155.

Featuring high-end protection and functions for an affordable price, it’s possible the most foolproof ATO system available.

The optical sensor and backup float switch offer you the best possible protect from common failures associated with other ATO systems.

But look, any on this list will get the job done for you. Some just require more attention and maintenance.

Your key takeaway is this: it doesn’t matter which ATO system you choose, if you don’t keep the sensor clean and free of obstruction, you’ll have problem.

Keep it clean.

Happy reef-building!

Last update on 2023-09-20 / Commissions Earned / Images from Amazon Product Advertising API

How to Choose the Best Auto Top Off System for Your Aquarium

There’s no one-size-fits-all method when it comes to choosing your automatic top off system. However, you can use this guide to identify which systems fit your setup and which features you need.

Start with the Sensor

This is the heart of your auto top-off system.

You have a few different types of sensors available, so let’s take a look at your options.

Float Switch

The original sensor, it’s inexpensive and your only option with moving parts.

Here’s how it works

Simple and effective, a Float Switch sensor uses a buoyant magnet to open and close the circuit as your water level changes.

The downside is, they’re prone to getting stuck.

Small marine critters, like snails, can get sometimes get on your float and prevent it from reading your water level properly. It’s possible for calcium and algae to buildup and prevent the float from moving, causing it to fail.

And since it’s a magnet, any others nearby could interfere.

Don’t get too downhearted. Most newer models have come up with solutions to these issues, like using plastic shields or back-up float switches to prevent failures.

Optical Sensor

Less prone to false readings or failure because it has no moving parts, Optical Sensors are a  prism-shaped water level sensor.

Here’s how it works:

If no liquid is present, an infrared LED light is shot through a prism into a light receiver to activate the pump.

Once your water level has risen, the light is unable to reach the sensor–shutting off the pump.

Temperature Sensor

Reliable and no moving parts, ATO systems with temperature sensors tend to be much smaller than the others, so might be one if you’re working will a small amount of space.

Here’s how it works:

Once you submerge the probe, it cools the sensor, as your water level drops below the probe, it’ll heat up–activating the pump.

Conductivity Sensor

Reliable and no moving parts, ATO systems with temperature sensors tend to be much smaller than the others, so might be one if you’re working will a small amount of space.

Here’s how it works:

Once you submerge the probe, it cools the sensor, as your water level drops below the probe, it’ll heat up–activating the pump.

Pro Tip: Whatever system you choose, the most important thing you do is keep the sensor clean and free of obstructions. Perform monthly checks to ensure your system runs smoothly.

The Reservoir Size

This is where you’re going to store the water for topping off your system.

Most hobbyists tend to use 5 gallon buckets or plastic jugs because they’re cheap, easy, and get the job done.

You just need to make sure your pump fits into the container.

If you don’t like the idea of using a bucket or jug, you could try a custom glass or acrylic container.

Old aquariums and sumps can make awesome ATO reservoirs. It may take a little planning, but the end result looks way cleaner and, in my opinion, “cooler” because it’ll blend nicely into your setup.

Now, if you have a large aquarium, a 5-gallon reservoir isn’t going to work.

You’ll have more evaporation, so you’ll need to replace more water.

I’ve seen other hobbyists use large polyethylene plastic tanks or large trash cans to great effect.

Here’s what it comes down to when choosing your reservoir:

Choose a water-sage container large enough to accommodate your tank size–big enough to hold 5-7 days of fresh top-off water. Any longer, and you’ll risk the water becoming stagnant and a build up of bacteria.

The Pump

Doing the job of transferring water from your reservoir to your tank, it’s another vital piece of equipment you can’t ignore.

Some ATO systems come with one included; some don’t.

Don’t sweat it if the ATO system you want doesn’t come with one. They’re easy to find and hook up to your automatic top-off system.

You have two main choices when it comes to pumps: dosing and submersible.

In my opinion, and most other hobbyists, dosing pumps are the best and most reliable option.

Why?

You won’t run the risk of a dry-run and pump water at a slow rate, making your setup safer because it won’t overflow your aquarium if you experience a failure.

Dosing pumps are not submersible, so you’ll need to mount them above your reservoir.

Then you have submersible pumps.

Easy-to-use, they offer a budget-friendly alternative to dosing pumps and have the ability to quickly deliver water into your aquarium.

Generally, ATO systems that don’t include a pump have a standard US 3-prong female plug to accept a submersible pump.

However, when it comes to submersible pumps, there is a downside you need to consider. Because they pump water quickly, you have less time to react before you tank overflows in the event of a failure.

And if you ever forget to top up your reservoir, you run the risk of your pump running dry–heating up, and potentially being damaged beyond repair.

Final Thoughts on What to Look For

Automatic top-off systems are possible one of the smartest investments you can make when it comes to your reef tank.

Not only saving you time, it helps you maintain a happy and healthy aquatic environment.