Originally Posted by Mebbid
Wow. A ton of suggestions and we don't even know the specific set up...
Assuming its a gravity fed ato with 1 hose leading to the sump I can see where your problem lies. A successful ato system needs two hoses. 1 in the bottom of the resivoir to feed water into the sump and another hose coming out of the top of the container. Keep the top hose where you want your fill line to be and the water line closer to the bottom.
When the water level drops below the top hose it allows air to flow through letting the water flow into the sump until the water level reaches the top tube.
The bucket has to be air tight for this to work.
Yikes! Gravity fed ATO. That's playing with fire... I mean water... Well, you know.
Upgrade to a proper ATO. :P
Also, not trying to nitpick here but, what you describe holds a very good chance of fouling your ATO water with a reverse syphon if great care is not taken. Maybe I am envisioning it incorrectly?