When using multiple LED lights over a reef aquarium, both the mounting height and spacing in between the LED lights have a big impact on the light spread inside your aquarium. With the ultimate goal of creating the most ideal environment for our corals, BRStv Investigates tackles a BIG question that many of us face when setting up our aquarium lights and the answer just might change the way you have your lights mounted over your tank! 

Even Light Spacing and PAR Distribution

While many of us tend to gravitate toward evenly spacing multiple light fixtures over the top of our reef tanks, this is often not the best way to achieve an even distribution of PAR.  We agree, the uniformity looks nice but this often results in an inconsistent spread of light. The overlapping light spreads and reflection of light off of the tank walls creates areas of varying PAR which can be problematic for supplying an evenly distributed spread of PAR over your reef.

The Experiment

The team sampled four different groups of light configurations over an aquarium measuring 48" x 24" x 24". We sampled eleven different brands of LED lights among the first three groups plus an outlier group using eight Aqua Illumination Prime HD LEDs.

  • 2x LED Fixtures
  • 3x LED Fixtures
  • 4x LED Fixtures
  • 8x Prime HD LED Fixtures

The results were recorded as a percentage that represents how close the outer PAR zones (left and right side) measures relative to the middle PAR zone.  A score of 100% would mean the average PAR on the left and right sides was equal to the average PAR we measured in the middle zone - a perfectly even distribution of PAR.  A score of 50% would mean the average PAR on the left and right sides was only half of the average PAR in the middle zone which ultimately means a hotspot in the middle of the tank with areas of much lower PAR on the outer edges. 

When the average PAR of the outer edges is within 75% or more of the average PAR measured in the middle, we considered this to be a passing grade.

2x LED Light Fixtures

2 Lights with balanced spacing 2 LED Light spacing results 2 light optimized spacing

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Looking at the results, we made some interesting observations. Both the Orphek and Phillips Coral Care do an incredible job of evenly distributing the light when mounted using a balanced spacing. All of the remaining lights performed best when mounted using the same optimal spacing with LED pendants moved closer to the outside edges, ultimately reducing the center hotspot.

This just goes to show how the different LED lights and their particular spread of light really do affect the way you should mount them. It also suggests that using the optimized spacing is more often a much better choice but is it is ultimately dictated by the particular lights your using. 

3x LED Light Fixtures

Balanced Spacing for 3 Lights 3 Lights Results

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With three light fixtures placed over the same 48" tank, we sampled five different brands of LED lights. The balanced spacing was set right at 12" even spacing between all three lights. The optimized spacing proved to vary slightly between each brand of light but in every case, one fixture remained dead center and the outside lights were moved closer to the edges.

We are starting to see that evenly spacing your lights across the top of your tank very well may not produce the best distribution of PAR. While the exact optimal spacing may vary slightly from fixture to fixture, there is an obvious benefit to creating a wider gap between the outermost lights and the center light. 

4x LED Light Fixtures

4 Lights Balanced Spacing 4 Lights Results

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When we tested four light fixtures, the optimal spread varied slightly again but certainly suggests the same approach.  You can accomplish a much more evenly distributed spread of light moving the outermost light fixture closer to the edges as opposed to evenly spacing them. This is because you're effectively reducing the center hot spot by eliminating or reducing the overlapping spread of light in the center of your tank that you get when they are evenly spaced.  

8x Aqua Illumination Prime HD LED Lights

Zoning AI Prime HD Output Intensity PAR Results

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This particular configuration poses a challenge because there really are thousands of combinations in terms of light spacing. We decided to approach this using zoning where each Prime HD was mounted to cover a 12" x 12" area over the 48" long tank. By reducing the output intensity of the centermost Prime HD fixtures to 40%, we reduced that center hotspot and were able to achieve a relative score of 84%.  The outermost average PAR zones came within 84% of the center average. 

What Is the Ideal Way to Space 2, 3, or 4 Lights Over A Reef Tank?

As a hobbyist, you don't really need a PAR meter to find the exact optimal spacing. Just follow the guidelines below to optimize the distribution of PAR across your aquarium based on the number of fixtures you are using. 

  • 2x LED Lights - Spread the fixtures farther apart, closer to the tank's edges by 2-3" as opposed to a perfectly balanced spacing. 
  • 3x LED Lights - One fixture should remain dead center with the outermost lights pushed closer to each edge of the aquarium by 2-3".
  • 4x LED Lights - Balanced spacing is acceptable but for a slight edge, just push all of the fixtures away from the center of the aquarium by 1-2".
  • 8x LED Lights - Mount them using a zoning approach where each of the fixtures is centered over its own "zone". Each zone should be the same exact size based on your tank's surface area.  Adjust output intensity to reduce the center hotspot. 

Learn More With BRStv: How many LED modules? Selecting and spacing reef tank LED lighting options - Reef FAQs