Is it a marineland hex? I have one, and thats why mine failed! It narrows at the base.
The tank is much larger in that regard and the common prefab screens dont fit, but you can replace them with that cheap plastic mesh stuff in whatever color you want ;D I like that the best, its also not as scary as the one-time cut thing!
GET LIKE 4 PIECES! XD LOLOLOL Youre gonna want it in that kind of tank, trust me! :P Oh, and its good to double them up to avoid the fish being able to see eachother.
Btw- sealant needs a tank that has been emptied and dried for a few days, cleaned with rubbing alcohol (if used) and needs to set for two days and cleaned after. Just as a warning if you have fishies in there already :B