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Auto Top Off

The water in our tank will start evaporating soon. This has two negative effects:

  • The salinity will increase because the salt stays behind. In a tank this small, this can make an impact in a matter of days.

  • The water level will drop and water could potentially stop overflowing into the sump. The pump will then drain the sump and will run dry, which is never a good thing.

Although an auto top off system, or ATO for short, is not strictly necessary it makes life a little bit easier for us and helps to keep the salinity and water level stable. We can walk over to the tank every other day with a cup full of RO/DI water and top it off manually but we must be disciplined about it.

If you go on a trip, the ATO can keep things working smoothly while you're away.

I started my first tank without an ATO and ended up having issues with salinity because of my lazyness. It affected several corals over a short period of time.


The ATO system begins with a container, or reservoir that holds fresh RO/DI water. This water will be pumped to the tank to make up for the evaporated water.

  • The size of this container is a matter of preference but I suggest 5 to 10 gallons for a tank this size. The larger it is, the less frequently it will need to be refilled - manually, by you.

  • You can use almost any kind of container you want: from a bucket to an old fish tank as long as it is made of materials that are safe and it is clean.

  • The reservoir will have a small pump in it, so it must have a mouth, or opening large enough for the pump to fit.

  • It has to be relatively close to the tank and be accessible so that it can be filled without making a big mess.

For this tank, I bought a 5 gallon jug just like the one pictured here.

Pump, Controller and Sensor

The second part of the system is a pump that will send the water from the jug into the tank. The pump is actually submerged in the water inside the reservoir. It is connected to a hose leading to the tank and a cable that supplies power to it from the controller.

Finally, one or more sensors or float switches are installed in the tank. When the sensor detects that the water level is low, it tells the controller to switch on the pump. This brings water into the tank raising the water level. When the sensor detects that the level is high enough, the pump shuts off and water stops flowing.


I would be remiss if I didn't warn you about the possibility for a mini-disaster when using an ATO system. If the sensor doesn't stop the pump, for example, it will drain the entire reservoir. This will lower the salinity in the tank and could even overflow, ending up with a lot of water on the floor.

This is the downside of ATO systems but, in general, they work well: saving us a lot of time and keeping things stable for our creatures.


For this tank, I chose the XP Aqua Duetto ATO which includes the pump, controller and two optical sensors as well as the hose and a bracket to hold it in place.

There are other similar systems so feel free to do some research and find what works for you. This particular product strikes a good balance between price and functionality plus I have a different ATO system from the same company in another tank and I'm very happy with it.

Here is a handy diagram from XP Aqua:

  • For starters, we have to clean the container. We're going to rinse it and let it dry overnight. Also, rinse the pump, the hose and the siphon break. Don't use any soap or other chemicals.

  • This container fits well in the cabinet, under the tank, so we'll place it there. This leaves room for some storage next to it as well as the shelf above it.

  • We have to cut the provided hose into two sections: one that goes from the pump to the siphon break and another that goes from the siphon break to the tank. The siphon break is a small connector with a tiny hole in it. When the pump is running, some water will come out of the hole so it needs to be inside the reservoir, or our stuff will get wet. I suggest the following: connect the entire length of hose to the pump and drop the pump in the container. Cut the hose about 2 inches below the point it exits the container's opening.

  • Attach the siphon break to the hose, making sure the tiny hole is pointing down and that the siphon break will remain above the water level in the reservoir.

  • Attach the rest of the hose to the other end of the siphon break and run it through the tubing holder, attaching it to the back of the sump.

  • Now, put the optical sensor on the back wall of the sump, so that it is halfway in the water. It has two sensors in the same housing and the bottom sensor is the one opposite the cable. It should be submerged. Use the magnet to hold it in place.

  • Connect the cable from the pump to the controller and the cable from the controller to the power adapter, but don't plug it in yet.

  • Now, fill the reservoir with RO/DI water, making sure that the water level stays below the siphon break and that the pump is resting on the bottom.

  • Finally, plug in the power adapter and don't forget the drip loop.

  • Take a look at the manual for the different ways the controller's LED can blink and take it from there.

Nothing should happen at first, but you can test it:

  • Lift the sensor out of the water slightly. The pump should turn on and water should start flowing into the tank.

  • Lower the sensor back into position and the pump should stop.

  • When it stops, make sure no water is getting siphoned back into the reservoir.

  • Look at the reservoir to make sure the siphon break is in place and the water coming out of it stayed inside the reservoir.

This system will keep our salinity and water level stable. It's a great time-saver!

Here are a couple of pictures of how it turned out. Since I use a 1/4" hose to refill the container directly from my RO/DI system, I didn't need a large opening: I can use the tiny hole you see on the handle of the container. If you're using a bucket to refill, this won't work.

You can see the tube on the left side of the sump and the sensor on the right side.


Check the water level in the tank when you walk by and give the optical sensor a quick wipe with your (clean) finger every few days.

If the controller's LED is blinking in unusual ways, pull out the manual and address the situation.

Check the water level in the ATO reservoir every few days. Once you get going, you'll have a good idea of how much water evaporates from the tank each week and you can fine tune your schedule.

By now, we have a few things that need periodic maintenance, like the filter sock, the fiber balls and now the ATO system, so we're going to start a maintenance document to remember everything.

Maintenance aside, we're almost ready to start cycling the tank but, before we do so, we need to get a water test kit and perform some initial tests.