very interesting, I believe light is a big factor in HC growth. Recently i upgraded my lighting system from 108w to 216w at the same amount of height above my tank and boom, GW! However, the HC was growing like mad, spreading faster than ever. Too bad i couldnt see the plants with my gw and uv sterilizer did not help. So i raised my lighting and so far the gw is slowly going away but my HC isn't growing as well anymore.
Although i fail to mention one thing is that during all this my CO2 diffuser broke so my CO2 distribution is varying from a ceramic diffusing to directly injecting co2 into my canister. I believe once i get my new ceramic diffuser in the mail HC should grow better but not as fast or lush as it was when the lighting was lowered.