The Complete Handbook of Aquarium Lighting

Chapter 1: Understanding Aquarium Lighting

Aquarium lighting is not a decoration. It is an environmental control system that directly governs photosynthesis, behavior, and even the biochemistry of the water column. If you get it wrong, no amount of filtration or water testing will make up for it. If you get it right, your fish, invertebrates, and plants will show it in their color, growth, and activity levels.

At its most basic, a light fixture converts electrical energy into electromagnetic radiation, a portion of which is visible to your eye as light. That radiation travels in waves, measured in nanometers. The human eye is sensitive to wavelengths roughly between 380 and 750 nanometers, which is why we call that range "visible light." But aquatic organisms do not share our visual spectrum. Zooxanthellae, the symbiotic algae living inside coral tissue, absorb light strongly in the blue range (around 420 to 470 nanometers) and, to a lesser extent, in the red range. Most photosynthesis on a reef occurs in water that is overwhelmingly blue because red and green wavelengths get absorbed and scattered by water within the first few meters. This is a fundamental point: a light that looks bright and white to your eye is not automatically the right light for the organisms in your tank.

Light in water behaves differently than light in air. Water absorbs energy quickly. The deeper the light travels, the less of it remains. In clear water, roughly half of the photosynthetically active radiation is gone by