I just reversed the flow through my undergravel filter(3 inches substrate) by attatching the outflow from my large canister filter to the down tube. After piling more substrate around the base of the downtube and agitating the substrate around the whole tank, which is planted with VHO lights, the water quickly cleared, there is no 'boiling' in the sand/gravel, and all seems well. I want to filter the particulates and detrius before it gets stuck in the substraight, and I wonder why more people don't do this. P