[Myka]

I bought a dehumidifier off Kijiji for $150. It keeps humidity around 35%. Before the dehumidifier the house would usually be around 50%, but sometimes up to 65%. My 90 gallon tank evaporates about 15 gallons per week. Before the dehumidifer it evaporated about 10 gallons per week. Understandably, lower humidity in the house does make the tank evaporate more.

Pertaining to the original question though, glass tops work better than a canopy (unless canopy is well sealed), and have the benefits of keeping temperature lower since you can cover the tank with glass, and still have fans to blow the heat from the halides away. However, I wouldn't suggest you seal the top of your tank in any way if you don't have a sump, and especially if you don't use a skimmer.

If you do seal up your tank, keep an eye on your pH with a calibrated digital pH meter as it may fall due to CO2 buildup if the tank doesn't get enough fresh air. Check before sealing it up several times during several days, then do the same thing after. Record the time and the reading, and compare readings at the same time as pH will fluctuate during the day, but should follow a daily routine.