My ATO began it's life as a Soda Ash bucket from my pool, and a water jug. I could not fit my reservoir under the tank, so this ugly setup sat next to the stand.
This setup gave me about ten gallons of water, and a siphon point that was at the level of the water in the bucket. This siphon point was important, because if it was above the level of my sump, the water would have siphoned.
I eventually painted the bucket black, and the water jug on the top made water additions easy....just like an old fashion water cooler. What eventually happened is that the lid of the bucket ripped.
This was fixed by cutting a 1/2 plexiglass piece to fit the lid.
Next came a cabinet that was on four small caster wheels and could easily be pulled out to change the water jug.
However, what this did was now hide the water jug and I wouldn't know when it needed to be replaced. So a horizontal float switch was installed in the side of the bucket.
This went through a relay to an LED that was installed in the face of my equipment cabinet...that green LED between the controllers.
When the water level drops below the float switch, the LED comes on telling me it's time to add a new jug of water.