I have a vivarium that is 24" high. There are two large wood pieces and a few water dishes and plants. The tank seemed too dark and the light fixture was pretty old so I got a new light fixture (24" long florescent style), but the tank still seems too dark, even after trying two different bulbs. Any ideas? Could it be that the light source is just too far from the bottom and diffused by the screen top? Or should I add another light source? Do bulbs typically lose strength over time even if they haven't burnt out?