Very happy with an item found on Amazon for about $60. They are a universal shape for mounting and should be 1 inch less than the height of the tank. Get the model that looks like a stainless steel pipe and has internal sealed electronics. The old style with a wire and a float are harder to install and the mechanical rheostat part is prone to fail due to poor (Chinese) construction. They come in a variety of lengths. We installed a 21" model on a 24" tall tank (140 gals) which left a 17 gal reserve when reading empty.