In a planted aquarium, light is the most vital element for healthy plant growth.
Plants use light as a source of energy - it is necessary for photosynthesis to occur.
Having live plants actually improves water quality. There are two basic lighting factors which are essential for a luscious aquatic growth. They are 'quantity' and 'quality'. In lighting terms, quantity is usually expressed in watts or more accurately lumens. The higher that wattage the brighter the light - pretty simple.