Green plants, which include aquatic plants, capture light on their leaf surfaces and perform photosynthesis. Photosynthesis is a mechanism with which plants synthesizes glucose from CO2 and water using light energy. Oxygen is produced as its byproduct. Photosynthesis is not only essential for the healthy growth of aquatic plants but it also supplies oxygen to fish and microorganisms. Lighting in an aquarium has the same role as the sun in natural ecosystems. Natural sunlight is hard to utilize in an aquarium depending on the place of installation or due to the fact that the intensity and duration of sunlight cannot be controlled easily. If strong, direct sunlight shines into an aquarium, algae may grow on aquatic plants. Phytoplankton also tends to proliferate and this results in green water. Therefore, artificial lighting becomes necessary. However, it does not mean that just any light will do. Besides providing light to aquatic plants for photosynthesis, lighting has another purpose of illuminating an aquarium beautifully for viewing as well. The colors of aquatic plants and fish do not appear beautiful with a red grow light designed for terrestrial plants or lights intended for indoor lighting. An aquarium requires the light that makes a layout appear beautiful and still promotes photosynthesis.
Strength
As a rule of thumb, planted aquariums should get 0.5-1.0 watt of fluorescent light per liter of water. Generally speaking, a 50 liter aquarium with standard dimensions will therefore require 0.5 watts x 50 = 25 watts. This rule has to be modified if you keep really high demanding or low demanding species, if your aquarium is very deep, or if your aquarium is really densely planted.
Incandescent or fluorescent?
Incandenscent lighting is still quite common, especially among beginners. There are many low demanding plant species that will do well with nothing but incandescent lights, but the problem is that incandescent lights tend to become really warm. This will affect the water temperature in your aquarium. Incandescent lights also consume a lot of energy and do not last very long. Investing in fluorescent lights can therefore save you money in the long run.
Color temperatures
When purchasing fluorescent lights from a well stocked lamp store, you may stumble over a wide range of different color temperatures. Different color temperatures are good for different purposes. If you are a novice plant keeper, stick to bluish (white) and yellow (warm) lamps.
Day length
Try to mimic the natural day length in the environment from which your plants hail. Many popular aquarium plants are tropical species and are therefore used to 10 hours of light per day.