If so, you dont want one there anyway. it restricts gas exchange. open top is how it should be without any cover directly over the tank unless it is something like eggcrate to prevent any jumping fish you may have
SO.......arcylic tops are out of date? I know mine is but living in CA and having the windows open year round helps I still would like to have and open top with MH (on the blue side) hanging from the ceiling. My tank is in an all-purpose room in a small townhouse so the "top" is nice. It is just too small (short) to really house the best type of lights.