Many aquariums require specialized lighting. Since discus fish require such meticulous care, it seems logical that these expensive fish might need complicated lighting arrangements. However, discus need only the barest of aquarium lighting. In fact, you have to avoid giving them too much light.
In the Wild
The captive lighting requirements for discus fish stem from the conditions of their natural habitat. Discus fish come from shaded rainforest rivers in the Amazon drainage. The dense canopies typically shade these rivers. Additionally, these waters typically contain lots of organic chemicals called tannins, which lower the pH, soften the water and make the water very dark. This type of water is called "blackwater" or "cola water." Discus fish are adapted to environments without much light.
In the aquarium, discus fish need very little light. Ambient light will likely provide enough illumination for the fish in most cases. Intense lighting can actually stress out discus fish and may harm the health of these delicate, expensive pets. However, despite their fondness for darkness, they do come from the tropics -- so make sure they receive 10 to 12 hours of light a day. Just make certain it's dim light.
Amount of Lighting
Aquarium lighting is usually measured in terms of "watts per gallon." This method has some drawbacks, since different types of aquarium lights are more energy-efficient, and this guideline doesn't take into account the depth of the tank. However, the guideline is useful for quantifying vague terms like "intense light" or "dim light." For discus fish, you typically want to provide about 1 to 1.5 watts per gallon of aquarium lighting. The specific type of lighting -- for example, fluorescents or LEDs -- does not matter as long as the lighting isn't too bright for the fish.
The low light levels preferred by discus fish have consequences for the tank. For example, most aquarium plants cannot live under such dim conditions. Paradoxically, most aquarium plants need much brighter illumination than this, while discus fish require a densely planted tank to feel safe. While a few low-light species can survive these conditions, it's easiest to just give up on live aquarium plants. Discus fish do well in tanks thoroughly stocked with plastic or silk aquarium foliage.