Because in most of the aquariums I've had the part where the bottom glass meets the side glass isn't perfectly straight but usually round because of the stuff they use to stick it together. Idk it's just what I've seen. ANd in all the tutorials they've said that you'll have to round the corners so that they'll fit together. Maybe I just need to find an aquarium that doesn't have that. I don't think I worded that very well.