In their natural habitat fish and plants receive intensive sunshine for 10-14 hours a day. However, outside the tropics the sun only shines that long during certain seasons and its strength is usually much lower. Daylight is understandably difficult to predict and control, particularly in the winter months. With too little light, plants will not flourish and the will fish feel uncomfortable becoming susceptible to disease. Dimly light tanks also harbour brown algae and plants soon wither and die. In direct sunlight there is a possibility of overheating the water and excessive growth of floating algae can occur. Consequently artificial, controlled lighting will be required for the aquarium.

Lighting is one of the most controversial and discussed subjects with aquariums. Not only does the range of lighting equipment available add to the confusion but also the bewildering range of terms used.

Most bulbs will identify their wattage. The wattage is a measure of the power the bulb uses, not how bright it is. Different types of bulb produce differing amounts of light per watt. By example, a fluorescent tube is around 4 times brighter than a household filament bulb of the same wattage, and a similarly rated metal halide bulb is around 8 times as bright.

Lux is a better term for describing how bright a bulb is. Lux is a measure of the amount of light a one lumen source (the SI unit equal to the amount of light emitted per second from a point source) can cast on an area of one square metre from a distance of one metre. Direct sunlight produces approximately 100,000 lux but under cloud cover this may drop to around 10,000 lux. Under water, because of its depth and colour, light levels will only reach between 100 and 5,000 lux.

Cryptocorynes sp. and other low light plants, do best at around 250 lux. Plants such as Amazon Swords (Echinodorus sp.) and other mid level plants prefer brighter conditions, around 800-1700 lux, whilst floating plants and species such as Vallisneria sp. and the Tiger Lotus (Nymphenaea lotus), require some 2000 lux or more. For most aquarium applications, approximately 13,000 lux at the surface is a good guide.

The colour of the light (not its spectrum, which will be described later) is measured by the temperature scale Kelvin (°K). Daylight measures around 5,500°K where as a filament bulb registers only about 2,700°K, you will observe that household bulbs look slightly orange. Higher temperature bulbs such as the 10,000°K metal halide bulbs used in marine applications favour a bluer colour and are unsuitable for freshwater use as this encourages algae.

The colour is important, as this is the factor that dictates how you aquarium will look. 5,500°K daylight bulbs are more expensive than standard 4300°K bulbs but the aquarium will appear more natural. The 4300°K bulbs produce more red light and give a yellow tint to the plants.

The spectrum is an influencing factor for inhibiting algae and encouraging plant growth. The spectrum describes the wavelengths that make up the light source. Visible light, such as daylight, contains all the colours of the spectrum from red to violet. However, fluorescent and metal halide bulbs for the aquarium usually only emit a few wavelengths, depending on the phosphors or rare earth used. Tri-phosphour bulbs are the best as these use three kinds of phosphorus rather than the usual two and produce three peaks in the spectrum more suited to limiting algae and promoting plant growth.

The energy contained in light is absorbed in the chlorophyll of plants but not all wavelengths of light are utilised with equal efficiency. This is logical. Plants do not use all of the green light they reflect it. This is why plants appear green. Many aquarists believe that only red and blue light is needed for photosynthesis. Whilst this is true for algae higher plants have evolved to use many wavelengths of light and the green and yellow wavelengths are not completely wasted.

Choosing a bulb of the appropriate spectrum helps to inhibit algae and promote plant growth. Algae are the oldest inhabitants of the planet. It is estimated that the first organisms, similar to blue-green algae existed over 3.5 billion years ago. The aquarium plants, with which we are familiar, developed much later.

Most algae species have always lived in water but through evolution, most of today's aquatic plants are descendants of plants that left the water and flourished as undergrowth in the forest swamps. The filtering of the forest canopy meant that these aquatic and swamp plants had to adapt to light with low blue and intense red wavelengths. Later, these plants eventually returned to the water.

Algae that existed in the oceans also had to adapt to filtered light but the seawater filtered this. In seawater, even at shallow depths (<5 metres), red light quickly becomes indistinguishable and at greater depths blue-green light prevails. Algae which migrated to freshwater still maintain the ability to see blue wavelengths better than red.

In fast flowing, shallow, sunny waters, brush algae is abundant. This is because the blue wavelengths, having not been filtered by the trees, is much higher than red. However, there are often rivers that are in full sunlight and are teaming with aquatic plants. Quite often this water is tinted brown with decaying organic matter and this colouring filters out a large proportion of the blue rays that algae enjoy.

Aquarium plants therefore respond most positively to light with reduced blue and increased red colouration in the lighting spectrum. It is important to note that simply choosing a light source with an increased red spectrum will not eradicate algae and favour plants. Light is only one factor that for controlling algae growth and promoting plant growth. Consideration must also be given to the substrate, filtration, fertilization and Carbon Dioxide and Oxygen levels.

The most popular method of lighting is the fluorescent tube. Fluorescent tubes are cheap in terms of initial outlay and power consumption. They also have the advantage that they can be concealed neatly within the tank hood. You will require a choke unit to light the tube and these are designed to operate with specific wattages (power ratings) of tube. Make sure that the wattage of the choke matches the wattage of the tube you intend to use. An under rated choke won't light a higher wattage tube, conversely an over rated choke will burn out a lower wattage tube prematurely.

Fluorescent tubes are often manufactured to burn for over 5,000 hours but their intensity diminishes with time. After one year a tube may only produce half its original luminance before it burns out completely and for this reason it is advisable to replace tubes annually. The use of reflectors, shiny metal plates fitted between the tube and hood, will reduce this effect and can improve the amount of light directed at the tank by as much as 80 percent. Painting the inside of the hood white or lining it with aluminium foil is a cheap alternative to purchasing a reflector.

Although the intensity of lighting required is dependent on the aquarium contents, particularly in terms of plants, generally a single fluorescent tube is not bright enough to penetrate tanks greater than 24 inches deep. (see choosing a tank for more information.)

The solution is to use either more than one tube or employ specialist lighting such as metal halide or mercury vapour lamps.

Metal halide and mercury vapour lamps are expensive to install in comparison to fluorescent tubes and usually must be suspended at least 8 inches above the tank. They can be incorporated in a hood but fans must be installed for cooling purposes and these can be irritatingly noisy. Although metal halide lamps penetrate deep into an aquarium they may produce too much light, which can encourage algae, and heat, in the upper regions of the aquarium. Floating or broad-leafed plants will be required to provide shade if this is a problem. The other disadvantage of metal halide lights is they don't bring out the colour of red plants very well.

Some lights are designed to promote plant growth, others to enhance the colours of the fish and in larger tanks different lights can be combined to cover both options. Multiple lights also enable dawn and dusk to be simulated by phasing the time lights are switched on or off. This good for the fish as a gradual reduction of light at night allows sufficient time for fish to retreat and to observe nocturnal species starting their "day". In the morning, a gradual increase in light helps reduce stress. As fish do not have eyelids, sudden exposure to light can cause sock. A simple timer socket can be used to control lighting automatically.

It should not be assumed that the more light in the aquarium the better. This is because the strong oxidising characteristics of intense lighting promote some types of algae. A solution to this problem is to reduce the lighting period from say 12 to 10 hours or better still, introduce a dark period during the day.

A successful lighting pattern could be a morning period of 4-5 hours light, followed by a dark phase of another 4-5 hours then a further 5-7 hour lighting period. During the "dark phase", the aquarium should never be in complete darkness but should receive some ambient light such as afforded by a window. Plants apparently enjoy these "siesta" periods and suffer no adverse effects, perhaps this period is representative of the storms that occur frequently in the tropics, where as algae loathe them.

One final point to remember. Electricity and water do not mix and although most aquarium lighting is designed to be splash proof I would recommend the use of a cover to protect lights against splashes.