Introduction: Simple ATO for Nano/Pico Tanks

About: Combat veteran who likes to tinker

Simple auto top off(ATO)for aquariums. I had a need for automated water top off on my nano reef tank. Problem was there are lots of ATO option out but none that fit my tank or needs exactly, so i decided to build my own. My main requirements were reliability, size, ease of use, and cost. Most of the ATO's available use float switches which are bulky and prone to sticking. Gravity feed ATO's are usually ugly and bulky. I chose to use a timer based ATO for its ease and reliability paired with an optical water level sensor for redundancy protection.

Step 1: Materials, Components, and Tools

List of parts I used

Digital 7day outlet timer - $9

120v to 12v power supply - $4

12v to 5v stepdown regulator - $5

5v relay - $5

Digital relay timer - $3.50

Optical water level sensor - $12

Enclosure - $6

Water pump - Used Aqualifter $14

Airline tubing - $3.25

Water container


Perfboard - $5

Connecting wire

Male header pins



Solder Iron

Wire strippers

Wire cutters

Rotary tool


Acrylic sheet - 4x4 sheet $3.75

Acrylic solvent glue - $7


Step 2: Circuit

I wanted the circuit to be timer based so the caveman drawing is what I came up with. I also worked with alot of parts I had on hand. So basically I use a timer relay to run the ATO pump, the optical sensor triggers a 5v relay to cutoff power to the timer relay which stops the ATO pump. So it is a timed circuit that uses the optical sensor as a failsafe cutoff. On the optical sensor circuit the 4.7k resistor can be changed to increase or decrease sensitivity of the sensor. Once i had it layout on a breadboard and working I transferred the sensor circuit to perfboard.

Step 3: Putting It Together

I used a cheap enclosure to house it all. I added a 120v GFCI outlet on the top lid to plug in the pump I had. Using a rotary tool i cut holes and notches for cables to pass then sealed them with silicone.

Step 4: Sensor Bracket

Not going to do specifics measurements as tank glass comes in all different sizes. I used 1/8 acrylic for the bracket because its easy to work with and fairly cheap. Cutting the acrylic, mark where you want the cut then using a acrylic cutting tool score the line afew times 4-5. Hold the acrylic on a table with the score slightly off the edge, then gently press down on the acrylic till it snaps creating a clean break along your score marks. After all your brackets are cut gluing is simple and fast. Using acrylic solvent glue like Weld-On #4, apply to one edge of acrylic the press and hold together. Setup time is about 4 mins. Drill a hole thru the base piece of the bracket, I used a 3/8 drill bit then finished the hole with my rotary tool with sanding drum bit( sensor was metric).

Step 5: Pump and Reservoir

Run one rigid hose form the pump to the reservoir and and a second hose from the pump to your tank. My tank is small so I did not have a need for a large reservoir. I used an old juice container I used for top off water anyway.

-If you have any troubles regarding any step or any point you can leave a comment Thanks