I don't think imbalance would have anything to do with it. If you have higher lighting, the plants are being driven to grow faster, thus needing more of each nutrient. With a fixed concentration of the nutrients the higher lighting would cause a shortage of some nutrients, causing unhealthy growth by the plants, with the less competitive plants not growing at all. The lower lighting, for 8 hours, shouldn't present any problems. If you were referring to 12 hours, it probably would encourage algae growth, but I believe most plants will grow for 8 hours with no problem.