I have tried many many setups for growing chaeto. From, high flow to low flow. From high light, to little light. I know what makes it grow. I've also cultured a ton of phytoplankton, and have been growing plants for over 15 years. I also majored in Biology. So I think I know what i'm talking about ;p
Oct, you're absolutely right, more light causes more growth. Actively growing tissue is a lighter green. Its pretty much the same story with most algae and plants. New growth is lighter in color. The way chaeto grows, results in the entire clump to be lighter green. Actively growing tissue has a lower desity of chloroplasts, causing it to be lighter in color. If the tissue stops growing, the chloroplast density is able to increase significantly, causing a dark green color. I have never been able to kill chaeto with too much light, believe me i've tried. The highest intesity artificial light source i've used was a 400 watt 6500K Iwasaki. The growth was stunning, even to me. I doubt anyone here is going to try more light than that, so I think it's safe to say you can't give chaeto too much light.
There are plants and algaes that will not do well under intense lighting, chaeto isn't one of them. Just like any plant/algae, you must acclimate to intense lighting. If anyone puts their houseplants out in the sun during the summer, they know that even plants that like full-sun, can be badly damaged from moving them from the shade of inside, directly into the intense light/heat of the sun. They must be gradually acclimated to the light. Perhaps this is where you have had problems Oct. BTW, what light were you using when you made the discovery that bright light kills chaeto? Just curious.
Trezona, Yes that is correct.