Salty Q&A: Bay Window Reef Lighting?

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

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


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!)

This shot illustrates just some of the 16 windows in the same room as my 125-gallon tank

This shot illustrates just some of the 16 windows in the same room as my 125-gallon tank

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.


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About Jeff Kurtz

Jeff Kurtz is the Co-founder/Editor of Saltwater Smarts, former Senior Consulting Editor for Tropical Fish Hobbyist Magazine, and the aquarist formerly known as “The Salt Creep.” He has been an aquarium hobbyist for over 30 years and is an avid scuba diver.


  1. I love natural sunlight on a tank but the many I have seen set up like this, mostly in stores always have a hard time trying to keep those panes free of algae. I know many people feel that as long as they keep nutrients low, algae won’t grow. There is a word I use for those people and that word is “wrong”. Algae just loves natural light as do I and Supermodels. Algae will grow in RO/DI water if you put it in sunlight. But it is a cool look, Good Luck

  2. Jeremy Gosnell says

    I’ve seen natural sunlight work in scenarios where reef keepers have installed specialized tubes that collect and magnify sunlight, spreading out over the surface of the tank. It can also work in greenhouse conditions, where corals are kept in large open troughs in greenhouses built to allow for total light penetration. Simply setting an aquarium near a bay-window and hoping enough light will move through the tank, keeping corals healthy, is likely hoping against hope. As Paul pointed out, those panes tend to get riddled with algae, eventually blocking any light penetration and requiring constant cleaning. I used to keep a reef in a sunlite room, and I loved watching the sunlight break through the glass, onto the reef – however it was a maintenance nightmare from an algae perspective. Now I keep my reef tanks in a basement fishroom and the only window is blocked with a light blocking blind.

    • "Caribbean Chris" Aldrich says

      Indeed, Jeremy! As Jeff mentioned, solar tubes have been utilized in some interesting ways. I’ve always fantasized about using them on a system. Will I ever actually do it? Likely not, but I certainly envy the ones I see done right.

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