Sunlight passing through a window generally provides very directional, odd angle lighting

Question: “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

Answer: 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 MORE