Only 18" lol darn it. It won't work for me then, I only have 9" on the sides of my tank bc of the overflows, unless I set it up vertically but I don't think many people have done that.How far across? The instructions come with a warning. It could blow your seams apart...lol I think you would need the 35w at the lowest setting, unless you have a wide front to back tank. The water might even flow out over the rim. Running it deep might help also.