the glass tops certainly keep the h2o from evaporating in my tank (never had them and added 10 gallon every saturday to my 120 gallon reef tank!!!!) it has been three weeks and only had to add very little :> )! also if your lighting is too close there is a chance of your bulbs getting splashed and breaking (once again I know from experience!) so I would suggest using them! also glass, if clear does NOT affect the quality of lighting ( sorry to dispute one persons theory!) we are of course talking light, the spectrum hues will pass through, only the prisms may bend toward the glass of the tank and not directly to the bottom of it! And, the temperature may rise slightly, but not a problem if you keep an eye on it!!!!