I guess I don't really see how your fill line managed to back up on you, unless you just put too much water in it. I would fill it all the way (cover the fill line till it's flowing if it backed up when you started it) and then after a couple days add some more water (the air takes a long time to completely get out, unless you assemble it underwater....even still, there's the chance that some air is stuck in a corner or something...) and keep doing that until the fill line doesn't get a lower water level anymore.