NOTE: This article was written by 30mini and has been moved here from the site Home Page.



Auto Top-Off, or ATO, systems have to be one of the most useful items to have when setting up any type of aquarium. To start, you will need to gather a few materials in order to get this to work properly. The list below is of must have items you will need in order to get this system to work. You will also need to figure out a way to hold the float switches in place where your water level will be. This will be discussed in more detail later.

List of Materials needed:

  1. (2) Float Switches (2 is recommended, 1 is needed)
  2. (1) 120VAC DPDT Relay
  3. (1) Pump of your choice
  4. (1) Cord to power relay
  5. (2) Crimp Wire Connectors
  6. (4) Crimp Connectors to fit onto relay


When buying the float switches donít forget to get the tubing adapter. The tops of the floats should never be submerged in water. The tubing adapter is used because when doing a water change, the water will go above the top of the float. You need to protect this area in some way. This is what the manufacturer recommends, in conjunction with some tubing, as shown in the diagram below. I recommend having a back-up float that will turn the system off in case the bottom float gets stuck open. Although only one is needed, I always use two.



The end result of this will be an easy way to maintain you water level using the simple logic below. The left shows that when the float switch is up, or open, the pump will not get power and will remain off. The right shows that when the float switch is down, or closed, the pump will get power turning the pump on putting water lost due to evaporation back into your system until it fills enough to move the float switch back to the open position.



2 Switch Configuration -

The first step to getting your top-off system created is to connect your float switches to the relay. Start off with your cord to power the relay and the float switch that will be used on the bottom. Take one wire from each and connect them together using your crimp connecter. It does not matter which ones you choose, as long as it is one from the float and one from the cord. Now grab your other float switch and take one of the wires from it and connect it to the other wire coming off of the first float. Take two of the connecters that fit onto the relay and crimp them onto the two loose wires. One of those wires goes to number 7 on the relay; the other wire goes to number 8. The below image is the relay to be used. Numbers will be next to each pin sticking out of the relay.



With your floats connected, it is now time to connect your pump to the relay. Somewhere on the pumps electric cord, preferably a section that is always out of the water, you will separate the cord in the middle giving you two separate wires. One side of the cord is going to be cut and stripped so that you can crimp on two of the connecters to fit into the relay. The below illustration may help you understand this a little more.



With your connecters on the pump cord, you are now going to plug them into the relay. One wire will go to number 4 on the relay; the other wire will go to number 6.

Plug everything in and test it. Make sure there arenít any sparks, and no touching contacts. When the float drops the pump turns on, and off when the float goes up. Also test to make sure that the top switch works in the same way. Remember that the top switch is a back up so that if the bottom one is stuck open leaving the pump on, the top one will turn off the pump.

Organizing -

I recommend getting a small box to put all of the electronics into. Whether itís a fancy one, or just something you have handy, anything will work. The only thing this box is for is to protect it against any water splashing that may occur, and to keep your hands away from any flowing electricity. Not sure if you have ever been electrocuted but I can tell you that it doesnít feel good! In addition to the box, if you can build an electrical panel that will hold your two plugs and give them both electricity, that would save you from having to take up two plugs on your powerstrip/controller.



Mounting -

Now comes the creative part. If you are handy with acrylic and can create your own using that particular material, then go for it! You can also use a few PVC pieces to achieve the same thing. What you need is a bracket of sorts to hold the switch in the area of your sump that holds your return pump. This is the only place in your system where the water level will drop due to evaporation. The bracket needs to hold the switch at the level that you want your ďrunning water levelĒ to be at. Remember that when the pump turns off that the water level will go up and cover your float. This is why you got the tubing adapter! The below photo is a good example of a holder.



Putting the floats in a place with little surface movement is best. If you are unable to put your float in an area with little to no turbulence, then creating some sort of guard will help to keep the float stable. This can be achieved many ways, the only thing that it needs to do is allow the float to travel up and down with the water level while keeping the waves away. A corner box, an old medicine container, a 2-liter bottle, whatever you have will work! Just make sure itís reef safe of course. Below is a great example. You can see the holes that the water will enter and exit allowing the float to move up and down, while keeping the waves and turbulence out.