When Should I Add Fill Light To an SPS Dominant Mixed Reef? | SR FAQ #7
Why add fill light in an SPS dominated mixed reef?
When acros are visibly shaded, browning out, showing stunted growth, or undesirable growth patterns adding strip or fill lighting usually fixes the problem or avoids it entirely. Fill lights brighten shadowed pockets and give SPS better, more even coverage so acros color up and grow more uniformly.
However, fill lights can make mixed tanks more challenging by reducing the number of low to mid PAR areas. That is, until the acros grow out and create new ones.
What fill lights do and what they don't
What they do: Fill lights remove dark spots, shaded areas and reduce hot spots, smoothing out the light across the reef tank. Over time that helps SPS corals like acros spread and color consistently rather than only thriving in the brightest peaks.
What they can cause: Because fill lights raise light levels in areas that were intentionally lower, they can make zoning for LPS harder. Corals that prefer lower PAR may end up in areas that are suddenly brighter, which can stress or bleach them if we don't plan for it.
Zoning is the best strategy for mixed reefs
Successful mixed reefs are all about zoning: creating high-PAR shelves and low-PAR refuges so both SPS and LPS can be happy. Adding fill light changes the map of PAR in our tanks, so we have to re-think where we place less light-demanding corals.
If we add strip lighting, we should also consider moving some LPS into remaining shaded nooks, or creating new lower-PAR areas with rock placement, overhangs, or deliberate shading.
Watch out for puck lights and hot spots
Small puck lights and narrow-beam fixtures tend to create intense hot spots and heavy shading. In tanks lit with these, fill lights can be especially useful because they even out those extremes. But they also raise the floor PAR across the tank, so plan coral placement accordingly.
If you add fill lighting to supplement focused pucks, it is typically necessary to dial the pucks down to maintain balance in the tank.
Measure first — PAR meters earn their keep
Before adding or rearranging lights, measure. A PAR meter helps us understand current intensity and how much fill light will change things. In many cases a PAR meter is just as valuable as the fill light itself, because it tells us where we need more or less light.
There are decent budget PAR meters available that give useful, actionable readings. Use them to map your tank: identify bright peaks, mid zones, and true low-PAR refuges before making changes. Check out this video for multiple $129 PAR meter options
Practical step-by-step when adding fill light
Map current PAR — use a PAR meter to establish a baseline across the rockwork.
Target problem areas — place the strip or fill lights over the shadowed pockets or stunted acro colonies, not necessarily over the whole tank.
Start low and acclimate — run the fill lights at lower intensity or for shorter periods at first. Increase gradually over several weeks while watching for bleaching or stress.
Re-zone for LPS — move LPS to remaining shaded spots or create low-PAR areas with overhangs or rock placement.
Monitor and adjust — continue PAR readings periodically and tweak light position, intensity, or coral placement as corals respond.
Final thought
Fill lights can be the simple fix our acros need, but they are not a free upgrade. They change the light landscape of the tank and force us to be deliberate about coral placement and zoning. Measure, plan, and adjust, and we’ll get both SPS and LPS thriving together.
FAQ
Should we always add fill light if acros are underperforming?
Not always, but often. If acros are visibly not doing well as they get larger fill light is a good corrective step. First measure PAR and check for other issues like flow, water chemistry, or nutrient problems before adding light.
Will fill lights bleach LPS?
They can if we raise PAR in previously low-light spots. Prevent stress by moving LPS to shaded areas, creating new low-PAR refuges, or ramping fill lights up slowly.
How do puck lights affect this decision?
Puck lights create intense hotspots and heavy shading between them. Fill strips usually help smooth those extremes, but they also elevate the baseline light level, so expect to do more zoning work for LPS.
Is a PAR meter necessary?
A PAR meter is highly recommended. It lets us make informed choices about where to add light and how corals will be affected. There are affordable meters that are accurate enough for hobby planning.
What’s the safest way to acclimate corals to added fill light?
Start the fill light at low intensity or short duration and increase gradually over days to weeks. Watch corals closely for bleaching, tissue recession, or color loss and back off if stress appears.
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