I had alway kept mine covered but was playing with the light the other day and noticed how much of a shadow the plastic hinge cast. So now I go topless and the only downfall has been slight evaporation, but not enough to be a problem. As hoppy said, definitely go with pressurised co2, and you will not only see a difference, but you will ask yourself why you hadn't done it sooner because once it is setup, there is no more mixing. My only other suggestion is the substrate. If you are using plain inert gravel, change it out to AquaSoil. This by far made the most dramatic change in mine.