I don't think canopy weight will be an issue as far as the structural integrity of the tank is concerned. Most commercially manufactured canopies are relatively light anyway...their appearance is deceiving. I maintain a 150 gal
tank at a diagnostic center that has a AGA (I think) Modern Cherry canopy with a 2x96w PC
lighting system installed in it. This is a 6' long canopy and I can easily lift it off the tank...I'd guess it weighs less than 20lbs. My guess is that it would take a huge amount of weight on top of the tank to cause any problems...much more than any canopy is going to weigh.