I don't know that the soil layer will become anaerobic under 5 inches of Eco Complete, but I do know that the soil layer will be of little benefit that far below the surface. Unless the tank has a depth of 18 inches or more, I don't think it will prove to be possible to maintain that big a slope with Eco Complete. Otherwise, I think I would try to put the rear soil layer on top of a sub-layer of Eco Complete, to get it closer to the surface. And, it would work better with at least 1.5 inches of Eco on top of the soil layer, to help keep the soil from showing up on the surface.