Lighting is very important to the health and growth of fish and plants. By far the best light for the aquarium is natural sunlight. It is difficult to match the intensity, and color spectrum of natural sunlight. Typically we keep our aquariums indoors, so the best we are able to do is to provide a few hours of natural daylight a day by where the aquarium is positioned. In most cases, we end up positioning the aquarium in a location where it receives no sunlight.
There is long running myth in the hobby about sunlight being bad for the aquarium. Sunlight is often blamed as being the cause of excessive algae growth. Algae only grows excessively when the aquarium water has excessive nutrients. In an aquarium with excessive nutrients, algae growth is actually making the aquarium ecosystem healthier for fish, by removing nitrate and phosphate that can suppress reproduction, growth, and the immune system. As long as you are able to keep the aquarium from overheating from sun exposure it is by far the best source of light for maintaining a healthy aquarium. Planted aquariums that receive several hours of sunlight every day often have very little algae growth.
Often when you buy an aquarium it comes with a standard fluorescent light with one or two rows of bulbs. Standard fluorescent lights typically are too low in intensity to help keep nitrates low or at 0 ppm. There are several alternative lighting systems that should be used instead of the standard fluorescent light, compact fluorescent (CFL), high output T5 fluorescent (T5 HO or HO T5), high intensity discharge metal halide (HID), hydrargyrum quartz iodide metal halide (HQI), and light emitting diode (LED).
The distance of the lighting system from the surface of the water should be considered when setting up the aquarium. The closer the light is the the surface, the greater the light penetration into the water. Lights placed close to the water level can transfer a lot of heat to the water, and the light distribution in the aquarium may be too focused.
The benefit from advanced lighting systems is that they can help bring out the natural colors of fish. The bright light helps intensify the colors in many species of fish, including tetras, livebearers, goldfish, loaches, gouramis, barbs, rasboras, and cichlids. Anyone that has kept goldfish in an aquarium for an extended period of time in a dimly lit aquarium, and then moved the fish to an outdoor pond, knows how bright light can intensify color.
For freshwater aquariums, bulbs in the 5000K to 6700K are generally best for plant growth, and a natural daylight look. 10000K bulbs have been used in combination with 6500K bulbs with good plant growth results. Actinic (blue light) bulbs and bulbs greater than 10000K should be avoided in freshwater aquariums since they are designed for coral that grows in deep water where only blue light is able to penetrate. A mixture of different bulbs can often improve plant growth of some species of plants, as it appears that not all plants respond equally to the same color spectrum.
Bulb life is a very subjective issue. Opinions vary as to how often CFL,T5, and metal halide should be replaced. Some say replace the bulb every six months, and some say when they burn out. There is no doubt, CFL,T5, and metal halide bulbs will lose both intensity and color spectrum as they age. My view on the issue is to replace the bulbs when nitrate accumulation appears to be increasing over time, or the appearance of the lighting has reached a point where it is not as bright as you prefer.
Metal halide (HQI and HID) are very bright, but they do emit a lot of heat. The light fraction from metal halide over an aquarium with an agitated surface creates a shimmering effect in the water column, very much like natural sunlight. You should have one bulb per 24 by 24 inches (60 cm by 60 cm) of surface area. For aquariums 24 inches (60 cm) or less in depth, 150 or 175 watt light are adequate. For aquariums 24 to 36 inches (60 to 90 cm) in depth, 250 watt fixtures are recommended. For aquariums 36 to 48 inches (90 to 120 cm) in depth, 400 watt fixture is recommended. Metal halide light are mounted approximately 12 inches (30 cm) above the water surface to optimally distribute the light. Metal halide is an excellent light for freshwater aquariums as long as you have a way of dissipating the heat they generate, either with a fan and/or a chiller.
Compact fluorescent lights (“U” shaped bulb) are much brighter than standard fluorescent lights. They come in many different sizes. Many aquarium kits are now coming with CFL fixtures built into the hood. CFLs are capable of keeping nitrate levels near 0 ppm in planted aquariums. The minimum recommended amount of bulbs is two bulbs running parallel to each other per 12 inches in aquarium width. CFL are good lights to use on aquariums up to 24 inches (60 cm) in depth. CFLs do generate heat, and may need the addition of a fan to keep heat transfer to the aquarium minimized.
High output T5 (skinny straight bulb) fluorescent lights are much brighter than standard fluorescent lights. They have become a very popular lighting choice for aquariums 36 inches (90 cm) or longer. T5 light fixtures come in many different sizes, but the most common are 24 inch/24 watt, 36 inch/39 watt, 48 inch/54 watt, and 59 inch/80 watt. T5s are capable of keeping nitrate levels near 0 ppm in planted aquariums. It is recommended that you have two bulbs running parallel to each other per 6 inches (15 cm) in aquarium width. In other words, if your aquarium is 12 inches (30 cm) wide, you should have four rows of bulbs. T5s are good lights to use on aquariums up to 24 inches (60 cm) in depth. The 54 inch/80 watt bulb is an excellent choice for 60 inch (150cm) long aquariums, like standard 100 gallon aquariums. T5 lights do generate heat, and may need the addition of a fan to keep heat transfer to the aquarium minimized.
LED lighting systems are the newest lighting solution for aquariums. They do produce a shimmering effect much like metal halide lighting. They produce very little heat under the light, but the electronics that power them can get very hot. If a LED retrofit lighting system is mounted in canopy, fans may be required to help vacate heat. Reviews on this type of lighting system have been mixed. How many LED lights you need for a given space is very subjective since all the manufacturers use different configurations. Individual LED lights cannot be replaced when they burn out as they are part of an array on a board. How well they work in controlling nitrate accumulation in comparison to other high output lighting systems is still unknown.