Can't really tell how deep the scratch is, or if its a crack, from that shot. If its really a scratch I wouldn't worry about it especially on a cube with the half inch thick glass. If it s a crack, going all the way or most of the way through I wouldn't use it at all. I have a 30 cube that was used for years as a seahorse tank at a lfs that had scratches similar to that all over it and they used it for years. I was able to buff the big scratches, 3 or 4 inch ones and about a half a millimeter deep, down with lense polish and a lot of time and effort but could never get it to look crystal clear. Probably could have done it with a lot more time, days, of buffing. So I gave up and turned it into a terrarium for my geckos.