the whole deal with tanks with plants and lights is finding the balance of light, and nutrients. pretty much no tank needs more than 8 hours of light. the thing is finding how long to leave the light on, pick a length of time, 6, 7, or 8 hours and monitor it for a week or two and see how much new algae grows. if you still grow a lot of algae, cut it back another half hour, etc. no matter what, you will always grow some algae, so find where you limit the algae growth, and still keep your plants growing.