It doesn't have a bald area where the light is most powerful, it merely has a different type of algae in that area making it look very different where the light is most intense. That algae is much lighter and greener than the black stuff that grows on the edges and is wiped off so easily. The circle in the middle seems to have some bulk to it and appears more filamentous than the slime that grows on the edges. I'll back the lights up a bit and see if there is a difference. If I see no change then I'll switch to the 100 watt bulbs (there's nothing in-between a 150 and 100 - not that I've seen at my local stores anyway.)
What's your take on the bubbles/foam??
Jeremy