I've had the light come on a few times. When it did I put over 19 gallons in, and the manual lists the capacity at 21.4 US Gallons, so I figure it comes on at around 2 gallons left.
I've had the light come on a few times. When it did I put over 19 gallons in, and the manual lists the capacity at 21.4 US Gallons, so I figure it comes on at around 2 gallons left.