if all you need is 24.5 inches you can use 24 inch background. what i did on my 175 is butted the background up to the underside of the black trim on the tank and smoothed it down the tank and taped it very neatly. with the rocks and plants you can't even see the little bit that it is short at the bottom. Take a look at my 175 in my profile and see what you think. worked for me. just don't measure the over all of the tank just measure the glass that you will see.
