I was woken up at 5:00 a.m. to the sound of a pump out of water (cavitating) so I got up to investigate to find the sump section of my refuge dry on my 220gallon tank aprox 5-7 gallons of water missing ( in the stand & carpet) my calcium reavcor line came out and stowly pumped out the water (return is a slow airline sized return (gallon or so an hour). If I would have had auto top off this would have gone un noticed till most likely 10:00p.m. and there would have been a lot more water to deal with and the salinity in the tank would be lowered. Just some thoughts about auto top off. Mainly auto top off hooked to a water line not a water container.