Keeping your salinity level stable is one of the most important things you can do as a saltwater aquarium owner.

To keep your tank water quality and chemistry appropriate for marine life, you'll need to replace the water that evaporates each day with new, fresh water (that has ideally been filtered through a reverse osmosis system).

The question is: how, exactly, are you going add that fresh water?

New hobbyists generally replace evaporated water (called "topping off") manually by simply pouring the fresh water into their aquarium whenever they get home from work or play.

Of course, this gets pretty old after a while. Who wants another daily chore? Plus, what if you're going to be gone for a few days and don't have any reefkeeping buds nearby to tank-sit?

Well, we are here today to tell you that your aquarium does not have to be a ball and chain that prevents you from getting away for a weekend at the beach or a holiday stay with your folks.

Yup, that's right: you can set up an inexpensive system to automatically top off your tank AS NEEDED, not just when it's convenient for you.

There are several different auto top-off (often abbreviated as ATO) systems available for aquarium keepers, with some more reliable than others (we'll explain later). And, although each ATO system utilizes different technology to achieve the same result—topping off your tank—there are some basic components most of them share. Namely the sensor(s), pump and a fresh water reservoir.

We will briefly explain what role each of these parts plays so you will have a basic understanding of how these systems work. Afterward, you should be armed with enough info to start shopping for your first (or next) ATO, although space, price and product reviews will all likely influence your final decision.

The Sensor

The sensor is the heart of your auto top-off system. Over the years, manufacturers have devised a few different methods to “sense” the water level and when it drops inside your aquarium or sump.

Float switch 

The original sensor in auto top-off systems. It's also the least expensive and the only option with moving parts. They are very simple and effective, using a buoyant magnet that opens and closes a circuit as the magnet moves up and down with the changing water level. 

The downside is float switches are prone to getting stuck. Snails and other animals may get on to the float and prevent it from reading your water level correctly. Calcium and algae buildup can also cause failure by preventing the float from moving freely. Since these systems utilize a magnet, other magnets nearby can cause interference with the circuit. 

The good news is most manufacturers have already come up with creative solutions to these commons problems, like using plastic shields and/or back-up float switches to prevent failure. The JBJ Automatic Top-Off and Tunze Nano Osmolator 3152 are two examples of popular ATO systems that utilize float sensors. 

Optical Sensor 

A prism-shaped water level sensor with no moving parts, which makes it less prone to false readings or failure. The optical sensor utilizes an infrared LED light that shines through a prism and is directed into a light receiver. If no liquid is present, then the light will be directed into the light receiver and the pump will be activated. When the water level rises, light is refracted through the water leaving little light to reach the receiver and the pump is then deactivated. You can find this method used on the Tunze Osmolator 3155 and AutoAqua Smart ATO

When Tunze incorporated this type of sensor into the Osmolator 3155, they created one of the most foolproof ATO systems to date. It uses both an optical sensor and a backup float switch to avoid the failures associated with other ATO systems on the market. As the water level drops, the optical sensor will turn on the pump. If it fails to turn off when water levels rises, the float switch is then used as a fail-safe.

Temperature Sensor 

These ATO systems feature a heating element and temperature sensor in an impressively small package. When the probe is submerged in liquid, it causes heat to dissipate quickly and the temperature sensor to cool. When the water level drops below the probe, it heats quickly, making the temperature sensor warm thereby activating the pump. 

With no moving parts, this is a very reliable option that rivals even the Tunze Osmolator 3155. The Hydor Smart Level and the Elos Osmocontroller both utilize variations of a temperature-based sensor.

Conductivity Sensor

This method utilizes two probes that, when immersed in liquid, complete a mild electrical circuit path through each probe to deactivate the pump. Once the water level drops below the probes, the electrical circuit is broken and the pump is activated. This system features no moving parts and are one of the more reliable sensor options. The recently released Innovative Marine HydroFill utilizes a conductivity sensor system.

No matter which system you choose, the most important factor in keeping your ATO system operational is keeping the sensor clean and free of obstructions. All of the aforementioned sensors should be cleaned and checked monthly to ensure proper operation.

The Reservoir

As the name implies, a reservoir stores fresh water used for topping off your system. The most commonly used types of reservoirs are 5 gallon buckets or plastic jugs. These are cheap, easy and get the job done. Just be sure that your pump fits into the container!

Another option is to use a custom glass or acrylic container. Old aquariums and sumps make great ATO reservoirs. Using them may require a little more planning, but the end result is generally a cleaner, more compact-looking setup. Most reservoirs can be concealed inside your aquarium stand or hidden behind a piece of furniture.

If you have a large aquarium, a 5-gallon reservoir is not going to cut it. A bigger tank requires a bigger reservoir since you'll be losing a lot more water from evaporation.

A Rubbermaid Brute Trash Can or large polyethylene plastic tank can fill this void. Top-off water containers for bigger tanks usually reside next to the tank, in a nearby closet, inside the garage or, if you're lucky enough to have space, a "fish room" (your basement, perhaps?). It is quite simple to run ¼” plumbing and small wires through a wall or door!

The main things to be concerned about here are choosing a water-safe container large enough to accommodate your tank to make automation worthwhile. Ideally, you want a container large enough to store 5-7 days worth of fresh top-off water—but not so large that the water will become  stagnant and harbor bacteria.

The Pump

A pump is a crucial part of your ATO system because it transfers water from your fresh water reservoir into your aquarium. Some ATO systems come with the pump included, such as the Elos Osmocontroller, Tunze Osmolator and AutoAqua Smart ATO.

Fortunately, it is easy to select, purchase and connect a pump to an ATO system for a custom setup if the model you're interested in does not include the pump.

The best and most reliable option is a dosing pump. They do not run the risk of dry-run and pump water at a slow rate. This makes for a safe-operating ATO system that will not quickly overflow your aquarium in the event of failure. These pumps are not submersible and will require external mounting above your reservoir.

The Tom Aquatics Aqua-Lifter is a vacuum-style dosing pump and one of the most popular (and affordable) top-off pumps available. I prefer peristaltic dosing pumps, such as the Aqua Medic SP 3000, because they provide a consistent flow, resist ware and will last much longer when properly maintained. They produce less vibration which equals quieter operation. If your container has a small opening, a dosing pump is optimal since it's external and will use tubing with a small diameter to pull the water out of your container.

The second option is a submersible pump, such as the ever-popular Cobalt MJ Series and Taam Rio pumps. Submersible pumps are easy-to-use, provide an inexpensive alternative to peristaltic dosing pumps and quickly deliver water to your aquarium. Most ATO systems that do not include a pump have a US standard 3-prong female plug to accept a submersible pump.

The downside to submersible pumps is they pump water quickly and are susceptible to dry-run. You have much less time before the tank overflows in the event of failure when compared to dosing pumps. If you forget to fill up your fresh water reservoir and the pump is activated, it will run dry, heat-up and, quite possibly, damage the pump beyond repair.

If you are tired of carrying buckets and pouring in cups of fresh water everyday... try an auto-top off system!

You will reduce daily tank maintenance, gain peace of mind that your water chemistry is stable and free up time to better enjoy your peace of the reef.

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