I am not going to vote since mine would be both I guess. I am building a canopy for mine that will include a Reto fit T5 fixtures. It will be semi-open in the back for air flow along with two cooling fans. I would say the advantage to open top is cooling of the water and somewhere for the heat to escape. The disadvangate is evaporation and I guess you could get a jumper. Closed tops will hold the heat, but will have less evaporation.