Good light, free, but hard to control
This is of course what fish and plants are used to, and it can hardly be
argued that this is anything less than the most natural. However, coaxing
enough sunlight into your aquarium, from the top, rather than through the
sides, throughout the whole year, can be problematical.
Sunlight is the certainly the cheapest way to illuminate an aquarium,
although it is unreliable and very difficult to regulate. This, of course is
subject to geographical variation. If you live in California and have a
skylight over an aquarium, you might be getting enough light. If however,
you live in an area that does not get a lot of sunlight, and your aquaria are
stacked in rows in a basement you will obviously need supplemental
lighting.
Very few people use sunlight as a primary lighting source, although it is
often used as supplemental lighting. Scrutinizing the photos of the 10,000
liter Dupla tank in the Horst and Kipper book _The Optimum Aquarium_
you may notice that besides having a number of powerful Metal Halide
lamps there are skylights for auxiliary lighting.
