Plants absorb light for photosynthesis primarily in the red and blue areas of the spectrum and reflect wavelengths they do not use, chiefly greens.
Plants absorb light for photosynthesis primarily in the red and blue areas of the spectrum and reflect wavelengths they do not use, chiefly greens. Bulbs with a low intensity in these wavelengths will fail to grow good plants despite how bright they may seem. Luckily, most aquarium bulbs now have a chart on the side of the packaging showing the amount of light of each wavelength that the bulb produces. Those with large spikes in the area of 430 nm to 460 nm (blue spectrum) and 645 nm to 665 nm (red spectrum) will provide the right colors to grow your plants well, and another spike in the 500- to 600-nm region (yellow/green spectrum) will produce a nice daytime effect in your aquarium.