Painting the back wall of your aquarium black will result in up to 33% less PAR in the back area of your tank. Rather than reflect the light back into your tank like the transparent glass walls, the black wall absorbs the light resulting in lower PAR values. We demonstrated how this works with a laser in our previous BRStv Investigates where we explored how reflection and refraction alter the output of our aquarium lights, specifically PAR measurements.

Watch BRStv Investigates: Have You Been Misled About Your Reef Tank Light’s PAR? Biggest Light Testing Mistake: Water vs Air

Lower PAR values across the back of the tank

We also see this phenomenon in all of our BRStv Investigates light testing videos where we consistently get lower PAR values on the back half of the tank. This is because the tank we use to take those measurements has a black background.

Since having a black or otherwise colored (blue) background is so common among aquarists, we are taking this a step further to find out exactly how and why that PAR drop occurs. We then set up an experiment to test the severity of this effect using different colored tank backgrounds including black, blue, light blue, white, and even a mirrored look to help all of you make the best choices for your own tank background.

PAR testing setup

PAR Measurements Using Different Colored Backgrounds

We set this up just like one of our BRStv Investigates lighting tests, taking a grid of PAR measurements from the top, middle, and bottom area of the tank.  We used the same Kessil A360X LED light mounted 8" from the water's surface for each and every test.  We used a 24" cube aquarium and recorded measurements using 7 different backgrounds.

  • Clear - no background
  • White
  • Mirrored
  • Black
  • Light Blue
  • Medium Blue
  • Dark Blue

PAR results using different colored backgrounds

After averaging all of the results for each using a total of 75 individual PAR measurements, we plotted the averages onto the bar graph above. The results were not surprising in that the black background resulted in up to 10% less average PAR throughout the entire tank. Black absorbs light, less light is reflected into the tank which results in less total average PAR.

We were surprised to see how the clear background acts just like a painted mirrored surface in terms of reflection. Without water in the tank, this clear panel would otherwise allow a majority of the light to pass through as opposed to reflecting it back into the tank.

Middle and bottom row average PAR results

By looking at the average PAR from the middle and bottom rows, we start to see how this PAR drop becomes more severe the deeper you go into the tank. In this case, up to a 28% drop in PAR toward the bottom of your tank if your tank has a black background. 

As a hobbyist, consider your aquascape here.  A rock wall or large pile of rocks covering a majority of the back wall will really negate most of this effect on your tank's PAR. On the other hand, if you have an NSA aquascape with open areas and islands, you're going to see these effects in your tank's total PAR.  Coral placed on the backside of your aquascape will simply receive less PAR compared to corals at the same depth on the front side of your aquascape. 

Clear back wall vs black back wall laser reflection

How And Why Does This PAR Drop Occur?

We set up a second experiment using each of our 7 different painted tanks and died the water neon green inside.  We then used our green laser, pointed at the back wall to see exactly how the light is reflected, refracted, and absorbed as it passes through the water and hits the back wall.  This should help to explain some of the results we got above during our PAR tests.

As seen in the image above, the clear or mirrored background reflects a strong laser of light. Suggesting a majority of your aquarium light will be reflected back into the tank.  The black background produces only a very faint reflection of light, exactly why we see the PAR drop in tanks with black backgrounds.

Clear vs white background laser reflection

Now we look at the white-painted background to find out exactly why the white background produced a drop even though we know white reflects light. Notice the halo around the point at which the laser touches the white background. In this case, we have to look at the difference between diffuse and specular reflection.

Specular Reflection Diffuse reflection

Specular reflection will not scatter the light and reflects the light waves at the same angle from the source. Diffuse reflection scatters the light and reflects the light in all directions which results in less light being reflected directly back into the tank. This same effect was seen on the light, medium, and dark blue tanks as well and was slightly less severe with each darker shade.

Which Color Tank Background Is Best?

We almost never consider white or light-colored backgrounds on an aquarium because it will be difficult to maintain a clean appearance. That leaves us with the choice of either a black or transparent background.

Black backgrounds help contrast the bright colors in your tank and help to create that infinite background effect, sort of like staring into the blue abyss as the ocean just turns deep blue then black. Transparent backgrounds simply reflect the backside of your aquascape, like looking at a mirror. Both will allow for coralline and various shades of nuisance algae to grow, so keeping them clean is going to require the same effort. Appearances matter in an aquarium, after all, it is a living work of art! 

The best logic would be to consider the style of aquascape you're going to use and how that will affect the PAR you need for the corals. A large rock wall or pile of rock up the back wall of your tank means the background color just won't matter that much at all. If your going with a more open or Negative Space Aquascape and will be placing corals to grow along the back wall, a clear or transparent background will provide you with better PAR compared to a black background

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