humidity levels??????

[jobin]

I recently bought 1 male and 1 female Phelsuma laticauda angularis 2 months ago. I have them housed in a large 60 gal. vivarium with 7 live plants, bark substrate, and 4 large peices of bamboo. For lighting and heat I use two 48" T8daylight flourecsant bulbs (i find that the normal daylight bulbs really bring out the color in my lizards as opposed to the "reptile" lights sold in pet shops) with 3 halogen puck lights for basking spots.At night I use a 75watt black light because it gets pretty cold without it. I spray the tank twice a day sometimes 3 times a day and I still cant keep the humidity at a high level. the temp during the day stays between 75-80 degrees and around 70 degrees at night. when I spray them in the morning the humidity moves up to 80% then drops throughout the day all the way down to around 40%. I've even seen it go as low as 30% some days. the drop doesnt seem to be affecting their health (at least, not yet). they eat on a regular basis and seem to be very alert and active. I've tried useing a small fogger but it proved to be too messy and it didnt last very long. Not to mention, it didn't raise the humidity very much at all. I covered half the top with plexiglass today. If this doesn't work what else can be done to keep the humidity level in the appropriate range???