Unlike other organisms that derive their energy from consuming organic material, plants obtain their energy from sunlight and atmospheric carbon dioxide. Sunlight provides a natural energy source which plants are able to harness through special cells in their leaves. The energy of sunlight is used to break down carbon dioxide and water, which can then be used to create glucose, a form of sugar, or storable energy. This process is called photosynthesis, and without it, complex life could have never evolved on Earth. Providing light in the aquarium is relatively simple but plants will do best under certain types of light. The spectrum, intensity and duration of light all affect plant growth in the aquarium.

The photosynthetic process
As mentioned above, light is vital for plants to carry out photosynthesis, the production of energy, or food, using sunlight, water, and carbon dioxide. The rate of photosynthesis is closely connected to a plants growth rate and ability to repair and reproduce. Providing other factors such as nutrient supply and environmental conditions are good, a plants rate of photosynthesis is directly proportional to the amount of light the plant receives. At low light levels, a plant may be unable to photosynthesis at a suitable rate, which results in slow growth and sometimes ill health. Light can be considered as a source of 'food' for the plants. Temperature also affects photosynthesis; a higher temperature increases photosynthesis whilst a cooler temperature will cause a decrease. However, an increase in the aquariums lighting and temperature will only benefit plants if they are able to obtain a suitable amount of nutrients and carbon dioxide. Therefore, if plants are not doing well in the aquarium, an increase in lighting may not always be the answer.
A single fluorescent tube lights most aquariums, in shallow (30cm or less) aquariums this may be sufficient for some plants. In most cases however, more lighting may be needed. The type of light is equally important as the amount of light.

Light spectrum
White light is made up of a number of different colours, each colour represents a 'wavelength' of light. The wavelengths, or colours of light are collectively known as the light spectrum. The spectrum can be seen in a rainbow, which is produced when raindrops 'spread' sunlight into its individual wavelengths. A light that appears slightly yellow, such as a normal household light bulb, can be considered as peaking in the yellow area of the spectrum, whilst a traffic light may peak in the red, yellow or green areas.
The light spectrum should be considered when providing light for aquatic plants. Each colour, or wavelength of light has a specific 'energy' and only certain wavelengths or 'energies' can be utilised by the special photosynthetic cells in plants. Green light for instance, is not used by plants and is reflected, this is why plants appear green. Red light has a 'long' wavelength and looses its intensity as it travels through water. Although plants can efficiently utilise red light, they receive a lower quantity of this light in nature. Blue light has a short wavelength and passes through water easily; both plants and algae use this type of light heavily. A good spectrum for aquatic plants would therefore peak in the easily utilised red areas and penetrating blue areas.

Duration of lighting
Simply increasing the duration of light in the aquarium may at first glance appear to be a good method of increasing a plants supply of light. Plants however, require a 'rest' period of darkness to function properly. If a plant were left in well-lit conditions 24 hours a day, it would most likely 'wear itself out'. Increasing the duration of light also encourages algae to grow, which is undesirable. For most plants, a period of 12-14 hours of light is sufficient.

Types of light
Most aquariums come with fluorescent tubes, which provide a low-cost, and easy maintenance form of lighting. For the majority of aquariums, fluorescent tubes are sufficient, although there are a number of different types to choose from. The different types of fluorescent tube all emit a different light spectrum. Some tubes are specifically designed for plants and produce a slightly yellow light whilst others increase the appearance of fish's colours (purple-blue colour), or provide a more natural white colour (full-spectrum). To create a good overall light for both the plants and the appearance of the aquarium, a combination can be used. A mix of one or two tubes designed for plants and a full spectrum tube usually produces an aesthetically pleasing light, which is also useful to plants.
Other types of lighting are also available for aquariums. The two most common of these are mercury vapor and metal halide lamps. These lamps are suspended above the aquarium (in these cases the aquarium does not have a hood) and provide a much higher intensity of light than fluorescent tubes. The lamps are expensive but have an intensity that matches several fluorescent tubes. The running costs are also far cheaper for lamps than for the equivalent number of fluorescent tubes. Lamps should only be considered for deep aquaria with many plants, or where the aquarium is specifically designed for plants.

Light loss in the aquarium
A great deal of light can be lost between the light source and the plants leaves in the aquarium. The effect of light loss is significant and may be as much as 50% or more of the initial output. Light loss can be reduced however by a number of methods.
Fluorescent tubes emit light in all directions and much of the light is absorbed by the aquarium hood or released through the glass. To recover much of this light, a reflector can be fitted which will direct all of the light downward, into the aquarium.
Once the light is heading in the right direction, it may have to pass through a cover glass or condensation tray. These are used in many aquariums to prevent water loss or condensation on electrical equipment such as the lights. The covers can quickly become dirty with water and algae deposits and will then absorb a great deal of light. Regular cleaning will minimise any light loss although in certain circumstances the cover can be removed altogether. Fluorescent lights run at a relatively cool temperature so are not affected by the odd splash of water or by condensation. If they are fitted with waterproof connectors, they can be used inside a hood without any protection (cover glass or tray).
Floating debris in the aquarium can also absorb some light so a good mechanical filter and regular maintenance will help to keep the water clear and reduce light loss through the water.

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