Summary: Point a fan at indoor hydroponic plants once they develop their true leaves.
Short: airflow
Airflow across plants is important because it helps with [transpiration][1], the process of water movement through a plant and evaporation off its leaves. The water is used to move minerals up the plant from its roots. Quoting Wikipedia:
Summary: Prevent hydroponic algae from growing by blocking light to any nutrient solution.
Short: algaegrowth
Algae will grow anywhere light is able to touch hydroponic nutrient solution. This won't be a problem if the solution is regularly flushed. In Kratky systems the algae will compete for resources with your plants and grow exponentially.