In a saltwater aquarium, maintaining a constant salinity level is extremely important to the health of your fish and corals.

Water constantly evaporates from the aquarium, while the salt in the aquarium does not. A replenishment system is required to replace the water that evaporates or else the salinity may quickly rise to unsafe levels.

One option is to manually add water to your aquarium each day. Unfortunately, this can be quite a strenuous task. In addition to lifting heavy buckets, adding a substantial amount of freshwater to your saltwater aquarium at once can be stressful for the tank inhabitants.

This is why so many hobbyists have found the use of an ATO or automatic top-off system to be extremely helpful. Today we will cover the various options available when it comes to choosing an ATO system and provide you with some insight that will help you select the right ATO for your tank.

Mechanical Float Valve

The most basic type of ATO systems are mechanical float valves that are used with a reservoir located above the float valve. Several companies offer these mechanical float valves, including AquaFX, Eshopps, Kent Marine and SpectraPure.

Using the force of gravity, water simply drains down from your reservoir into the float that is mounted in your sump or aquarium. Once the desired water level is reached, the float simply closes off the water line.

Eshopps Auto Top-Off Reservoir Tank

Mechanical float valves are very affordable. However, salt creep, calcium build-up and various other obstructions can cause float valves to fail. Additionally, it can be difficult to find a suitable place to mount the water reservoir above the float. For these reasons, it is best to use mechanical floats on a freshwater tank or in conjunction with your RO/DI system to keep your freshwater reservoir full at all times.

The second group of ATO systems are ones that utilize float switches. These moderately-priced systems have an electronic float switch(es) that are used to trigger a relay which then controls power to a water pump that is located in your ATO reservoir.

Marine Depot ATO System w/ Pump and Magnetic Sensors

Our Marine Depot ATO systems are one of the most affordable options and are available with a single float or dual floats. All the necessary parts are included so you are ready to go out of the box. One of the great features is the acrylic guard around the float switch. This guard helps reduce turbulence and helps prevent snails or other critters from obstructing the operation of the float switch.

JBJ ATO Water Level Controller

The JBJ ATO features dual float switches that can be set up in multiple configurations, including dry-run protection for the top-off reservoir. It has a standard outlet so you can plug in just about any type of powerhead or water pump to deliver water from your reservoir into your aquarium.

Tunze Osmolator Nano 3152

Next is the Tunze Osmolator Nano. The Osmolator Nano includes a single magnet mounted float switch with a low-voltage DC water pump. It is simple, effective and great for smaller aquariums

The last group are the more costly ATO systems that feature more advanced sensors. These ATOs use higher-end sensors or multiple types of sensors to make the system as fail proof as possible.

Tunze Osmolator Universal 3155

The Tunze Osmolator 3155 is our best-selling ATO system. It uses an accurate optical sensor with a mechanical float as a backup. The combination of the two sensors makes it extremely accurate and nearly fail proof. The included low-voltage pump is also replaceable and fits perfectly into 5 gallon water jugs.

AutoAqua Smart ATO System

Our next best seller is the AutoAqua Smart ATO. This ATO uses an advanced infrared optical sensor that measures the water level accurately. Additionally, the integrated Auto Feedback Security (AFS) algorithm programmed into the unit will learn and detect overfill as well as sensor failures. Its small size and the magnetic sensor mount makes installation a cinch. The kit also includes a small water pump that fits in just about any reservoir.

Hydor Smart Level Control ATO System

The Hydor Smart-Level ATO uses a temperature sensor so there are no moving parts. Having a temperature sensor also means that you don't have to worry about snails climbing on the sensor or algae and calcium build-up. A slight drawback is that it does not include a water pump and has a larger water level variance of ½" to ¾". Still, it is one of the more affordable advanced ATO units.

Innovative Marine HydroFill Ti ATO Controller

The Innovative Marine HydroFill ATO uses high-tech conductivity sensors. The electrodes in the sensors are inert, so there is no chance of rusting and are they are very durable in a saltwater environment. They can be used with just about any water pump—or you can purchase the awesome HydroFill pump which is a self-priming diaphragm pump (that means it can sit above the water reservoir and will draw water up into the pump and then into your aquarium).

An ATO system is one of the smartest investments you can make for a reef tank. Not only does it save you time, but it also helps ensure you maintain a stable aquatic environment. Plus, with so many options available, it is easier than ever to find a system that fits your budget and the needs of your aquarium.

Thank you for allowing us to share our experience with you. If you found this article and the accompanying video helpful, please like, share and subscribe to show your support!

Until next time, take care and happy reef keeping.

> Watch our Auto Top-Off System Video Playlist

> Read 10 Helpful Ways You Can Use MJ Pumps

> Read Aquarium Top-Off Systems: What is the Difference?

> Read Fed up with carrying buckets? Try an auto top-off system!

> Read Tunze Nano Osmolator 3152 Product Review

> Read Hydor Smart Level Control ATO System: Product Review