greenstuff, i have allways used glass between light and tank ,if for no other reason but to keep reflector clean. as far as keeping it clean i think that should be figured in watts per gal, meaning a few more watts to make up for dirty glass. you can make that glass sparkle and the next day it looks terrible from the fish spashing during the night. i would rather be safe with glass between then sorry if light falls into tank for some unknown reason. glass on a 75 or 90 gal tank is large, broke one, then went to expensive plastic. works fine. glass over a tank also helps to keep fish from jumping out and some think that it helps to keep co2 in tank. hanging strong lighting above a tank without glass would be the most desirable senerio,but you need a lot of room for that. regards, cornhusker.