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.
Two 150 watt HQI Metal Halide lights over a 30 gallon tube tank.
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.
Three 65 watt Compact Fluorescent Lights over a 60 gallon cube tank.
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.