When mounting an LED fixture over the top of your aquarium, the distance between the source of light and the water's surface significantly affects the light's performance in the way of coverage. If you mount the light too close to the water's surface, most lights create a significant hot spot in the center of the aquarium with a drastic drop-off in intensity toward the edges.

As you move higher up, away from the water's surface, the light distribution becomes more even, reducing this hot spot effect. If you go too high, however, the light bleeds over the edges of your aquarium and is effectively wasted. 

When determining the optimal mounting height, you must consider this trade-off between even light distribution over the coverage area (hot spot) vs. light loss over the edges of your aquarium (efficiency).

PAR measuring grid

The Experiment

We used our custom PAR measuring grid to test 50 different lighting options at varying mounting heights over a 24" cube aquarium. The ultimate goal was to come up with BRS Recommended mounting heights based on the PAR data collected.

Hot Spot

Minimum mounting height - Measuring Hot Spot severity to gauge distribution 

The severity of a hot spot increases the closer you mount the light to the water's surface. We measure the severity of a hot spot by taking a grid of PAR measurements at the top of the tank. We then compare the average PAR in the center (directly under the light source) to the overall average of the entire grid. The PAR variance between the center hot spot and the overall grid will tell us how well the light is distributed over the coverage area.

  • Hot Spot Reduction Goal: No more than a 30% variance between the average PAR of the center hot spot and the overall average. 

We started at a mounting height of only 1" from the water's surface and then moved the light up incrementally, recording a PAR grid at every inch. We determined the minimum acceptable mounting height to be where the light no longer creates a severe hot spot as indicated by a variance of 30% or less. 

Minimum Mounting Height
Maximum Mounting Height

Maximum mounting height - Measuring light loss to gauge the efficiency

As your light source moves up and further away from the water's surface, the conical spread of light becomes larger. Based on the exact size of that conical spread of light, there will be a point at which the light spills over the edges. This light loss over the edges results in a decrease in PAR inside the aquarium.

To measure efficiency, we used the same PAR grid to calculate the total average PAR inside the tank and recorded measurements, inch by inch, as we moved the light up, away from the water's surface. This will give us an indication of at which height the light is most efficient. We can also analyze the data to show the degree of light loss that occurs as the light source is elevated away from the water's surface.  

  • Efficiency Goal:  At least 70% of the average PAR is retained inside the aquarium; no more than 30% light loss. 

The maximum mounting height is the greatest height where at least 70% of the average PAR is retained inside the tank. Retaining anything less than 70% of the average PAR is mounted too high up and deemed an unacceptable level of light loss. 

What is an "optimal" mounting height?
When the minimum mounting height does not exceed the maximum mounting height. Mounting your light within this range will create optimal light distribution and retain no less than 70% of the total average PAR.

What is a "sub-optimal" mounting height?
This is when the minimum mounting height exceeds the maximum mounting height. The lighting option cannot achieve a less than 30% reduction in the center spot while also retaining 70+% of the total possible PAR.

Some manufacturers prioritize distribution, some prioritize efficiency, and some offer a forgiving solution where there is a range of mounting heights that produce "optimal" results. This becomes obvious when studying the PAR data graphed in the video, especially in the case of the "sub-optimal" mounting heights. In the case of a sub-optimal mounting height, it's all that more important to be precise in the mounting height to get the most out of your lighting option.    

BRS Recommended Mounting Height

BRS Recommended Mounting Height

When coming up with the BRS Recommended mounting heights, we chose the maximum mounting height because this represents the height at which you can achieve the best possible distribution while retaining a reasonable level of light retention. 

It should be noted that most lighting manufacturers provide a recommended mounting height range. In most cases, this is very similar to the BRS Recommended height but is not always an exact match. While our heights were determined using our criterion and PAR data, lighting manufacturers are not held to any standard for calculating their recommendations. The BRS recommended mounting height is defined as inches from the water's surface. 


What We've Learned

Incorrectly mounting your light can not only result in wasting light but can also create hot spots of dangerously high levels of PAR that will burn corals. You can also wind up with a very inefficient install and waste light that could otherwise be harnessed at the correct mounting height.  These are very common mounting mistakes that should not be overlooked and ultimately using a PAR meter is the best tool to achieve mounting success.  

The optimal mounting height will change based on the shape of your aquarium and the configuration of your primary lights. Our test was performed over a 24" cube and a rectangular-shaped aquarium would have produced a different data set, resulting in different optimal mounting heights. 

The optics and engineering of a particular light heavily influence the ideal mounting height. Most small profile lights with very focused light sources are not forgiving and you have a very small mounting height window where both efficiency and hot spot reduction are optimized. Larger form factor light configurations that prioritize distribution are more forgiving in the way of mounting height and make it easier to achieve ideal light coverage.

LED lighting technology has evolved and there are modern LED lighting solutions that actually perform better than T5 and metal halide lighting of days past. It's just a matter of applying this LED technology correctly.