Glass or Acrylic?
 

I personally don't use a lid at all on my aquarium. Those lids just trap heat in the tank. IMO leaving the tank lid off leaves a greater surface area for evaporation; a natural cooling process.

If you absolutely have to have a lid on top of your tank, use acrylic. It's easier to cut, and less likely to break. And if it does break, it isn't as likely to cut you.

No, you didn't misunderstand. I have the same light, in 72". I have used both the 72" version and the 36" version over an open tank. I think that for insurance/warranty reasons, they state this in the instructions. I have personally had no problem with keeping this light over an open tank.

Awesome! I was trying to figure out what it used wattage wise per day, 8 hrs due to having a Crocea clam and read that, so I thought it was must...Will pass on covering it up....Thanks for the Feedback...:)

good thread, thanks for the info