Lighting...it's real name is light defuser pannel. Heres a picture of a divider made out of it. (Theres one underneith every tank we own....just in case.
We've toned the aggression down quite a bit and don't have ANY of the tanks divided at the moment...The fish in the picture have moved in with my 20" aro and another pbass about the same size. Egg crate also makes great filter parts....tank tops...I even used some when I made my testing kit...lol
or instead of egg crate use a plexiglass a little expensive than egg crate but look's much better and will not hurt or damage your fish available in Lowe's and Homedepot too just measure the inside of your tank and let them cut it labor is free anyway.