I don't have a top on my tank either. I fashioned a barrier out of florescent light covers to keep fish from jumping out. Other than that it is completely open.

Not keeping a top on the tank has helped a lot with heat dissipation. One of the cons is that I have to top off the tank every other day.



Here's a crappy image of my tank, but as you can see the "guard" that I made for the top of the tank does not attenuate the light: