Too much light intensity prohibiting or stunting photosynthesis.
I was reading an article on this subject a while back and as I am almost done with my new canopy, I was debating whether a 400w 10,000k SE MH in addition to 4x 36" T5 Actinics on an IceCap 660 would be too much for a 36x24x24 tank.
Thanks for your input,
Ivan