This is an experimental fix for brown/black growth, and also for extending the life of the bulbs past 3 months:

1. Use bulbs with twice the wattage as recommended; so use 2 real watts per square inch (6.25 sq cm) of screen. Thus a screen 10 X 10 inches = 100 square inches would get 200 total real watts of light instead of 100 watts.

2. Run the lights for half as many hours as recommended; this would be 9 hours instead of 18.

3. Around 3 months, when the growth starts to get darker (because the bulbs are getting weaker), start increasing hours until it grows green again. When you reach 18 hours, it's time for new bulbs.

This does a few things: The first is obvious; you will get longer bulb life as long as you remember to increase the hours when you get to 3 months. But stronger light can also turn black/brown growth into green, no matter how high the nutrients are, if the light is strong enough. The trick is just to not burn the algae; thus the hours have to be less.

When the light is strong (compared to the nutrients), more of the growth is physical algae, and it's also more green (less proteins). When the light is weak (compared to the nutrients), more of the growth is DOC and dark physical growth (more proteins).