I’m setting up my first reef aquarium and want to keep my approach as natural as possible. The room where I plan to place the tank has a really big bay window in it, and succulents and other sun-loving houseplants really thrive there. Is there any reason I can’t take advantage of all that natural sunlight for my corals instead of using crazy expensive artificial lights?” – Submitted by CZ
Though you would save a bundle if such a plan were feasible, I would discourage relying on window lighting to illuminate your reef system for several reasons. First, the amount of sunlight passing through the window is going to change throughout the year as the sun’s position shifts and the days get longer or shorter with the seasons. That won’t bode well for tropical corals, which demand 10 to 12 hours of direct sunlight per day.
Second, the sunlight passing through the window will reach the tank at an odd angle and from only one direction. So even if you could get enough sunlight of sufficient intensity to pass through the window and onto your tank on a consistent, year-round basis, your light-hungry inverts would always be shaded on one side.
Third, placing an aquarium too close to a window—especially one that lets in a lot of direct sunlight at certain times of year—can make it difficult to maintain a stable, appropriate water temperature, which is stressful to the inhabitants. (I can attest to this since my 125-gallon tank is situated in a room with 16 windows!)
Last but certainly not least, depending on your work schedule and other factors, relying on natural sunlight to illuminate your reef system may severely limit your viewing pleasure simply because the tank will be dark whenever the sun isn’t shining.
All that being said, there are reef hobbyists who use natural sunlight to illuminate their home aquariums. However, they don’t typically achieve this in the manner you’re considering. Rather, they use special solar tubes that capture the sunlight on the roof and direct it down onto the aquarium (such as this tubular skylight). I can’t say whether solar tubes would be an appropriate option in your case, because there are myriad factors to consider before endeavoring to implement such a system, but they can be a means to save a lot in ongoing utility costs—at least after initial installation expenses have been recouped.