Lighting Science: Spectrum, Intensity, and Photoperiod Must Agree
Lighting needs depend on livestock, tank depth, nutrient balance, and whether the goal is low-tech plants, demanding aquascapes, soft corals, or higher-demand reef species.
What lighting decisions affect most
Lighting variable
| Lighting variable | Why it matters | Safer approach | Common mistake |
|---|---|---|---|
| Spectrum | Different wavelengths support growth and viewing in different ways | Choose a fixture aligned with the tank goal | Judge only by how vivid it looks to you |
| Intensity | Too little can stall growth; too much can feed nuisance algae or stress livestock | Increase gradually and observe response | Max out power on day one |
| Photoperiod | Duration compounds the effect of intensity | Use a timer and keep it stable | Change hours constantly by mood |
| Livestock match | Plants and corals do not all want the same output | Research the specific organisms in the tank | Treat all planted and reef systems alike |
Lighting control rules
| Use a timer to lock in the daily schedule | |
| Adjust one lighting variable at a time | |
| Match the fixture to tank depth and livestock | |
| Expect algae signals to reflect total balance, not only the bulb | |
| Keep dark hours truly dark for rest | |
| Run high intensity and long hours immediately | |
| Copy a reef or aquascape schedule without matching the rest of the system | |
| Assume expensive lighting removes the need for nutrient balance |
The most useful lighting question is not 'How bright can I go?' but 'How much light can this whole system actually support consistently?'
When algae explodes after a lighting upgrade, the light may be the trigger—but the underlying imbalance usually includes nutrients, CO2 strategy, or maintenance discipline too.
Aquarium lighting is more complex than watt counts suggest. The light that plants and corals need is defined by three variables: spectrum (the wavelengths of light provided, measured in nanometers), intensity (the actual photons delivered at the substrate or coral surface, measured in PAR -- Photosynthetically Active Radiation in umol/m2/s), and photoperiod (the duration of the light cycle). A light that looks bright to human eyes may be useless to plants if it lacks the right wavelengths; a dim-looking light with the right spectrum can support a lush planted tank.
| Technology | PAR Output | Best For |
|---|---|---|
| LED (quality) | High + programmable | All applications; 50,000+ hr lifespan; ramp scheduling |
| T5 Fluorescent | Moderate, consistent spread | Planted tanks; even coverage; replace bulbs every 12 months |
| Metal Halide | Very high + shimmer effect | High-end SPS reef; high heat and running cost |
Spectrum: What Plants and Corals Actually Use
Chlorophyll a and b (the primary photosynthetic pigments in aquatic plants and zooxanthellae, the algae living inside coral tissue) absorb light most efficiently at two wavelength ranges: 430-450 nm (blue/violet) and 640-680 nm (red/orange). A light providing primarily green and yellow wavelengths (450-600 nm, which appears bright white to humans) has poor photosynthetic efficiency even at high intensity. Dedicated plant lights and reef lights are engineered to emphasize these absorption peaks.
The Kelvin rating on aquarium lights indicates the color temperature of the light (how blue or yellow it appears) but does NOT directly specify spectrum or intensity. A 10,000K bulb appears whiter-blue to human eyes and is commonly used in reef tanks; a 6,500K bulb appears slightly yellow-white and was long standard for planted tanks. However, Kelvin is a poor specification for comparing light quality -- PAR data and spectral graphs are far more informative. Modern planted tank and reef lighting manufacturers publish PAR data and spectral output graphs; use these to compare lights rather than relying on Kelvin ratings.
PAR: The Measurement That Matters
PAR (Photosynthetically Active Radiation) measures photon flux density in the 400-700 nm range that plants and corals use, in units of umol/m2/s (microeinsteins per square meter per second). PAR meters cost $200-$500 new (also available for rent or loan from some reef clubs) and allow direct measurement of light at any point in the aquarium.
PAR requirements by tank type: Low-light planted tanks (crypts, java fern, anubias, moss): 10-30 umol/m2/s at the substrate. Medium-light planted tanks (stem plants, swords): 30-80 umol/m2/s. High-tech planted tanks: 80-200+ umol/m2/s. Reef tanks: Soft corals: 50-150; LPS: 75-200; SPS: 200-600+ umol/m2/s.
Photoperiod: How Long to Run the Lights
Most aquarium plants and corals adapted to tropical latitudes receive approximately 12 hours of light near the equator. Slightly shorter photoperiods (8-10 hours) reduce algae growth in planted tanks while still supporting plant photosynthesis. Longer photoperiods (12-14 hours) are sometimes used in high-tech planted tanks with CO2 injection.
A key insight: many planted tank and reef keepers use a "ramp" cycle -- gradually increasing intensity at the start of the photoperiod, reaching peak PAR for 6-8 hours midday, then ramping down before lights-off. This simulates natural daylight cycles, reduces the shock of abrupt darkness, and can help with dawn/dusk spawning cues for fish and corals. Programmable LED controllers enable this easily; T5 and metal halide fixtures do not.
Technology Comparison: T5 Fluorescent, LED, and Metal Halide
T5 fluorescent tubes: well-understood, consistent spectral output, relatively inexpensive. Significant heat generation; bulbs must be replaced every 12 months as output degrades before the tube visually fails. PAR output is moderate but consistent across the footprint -- T5 arrays spread light evenly, which suits planted tanks well.
LED: the current standard for most applications. Efficient (lower heat and electricity costs), long lifespan (50,000+ hours rated), programmable. PAR output has increased dramatically; quality LED fixtures now rival metal halide for reef applications. The quality gap between cheap and quality LEDs is large -- budget LED bars ($20-$80) rarely deliver adequate PAR for demanding applications; quality reef and planted LEDs ($150-$500+) deliver validated PAR. Verify PAR data before purchasing for serious planted or reef applications.
Metal halide: the traditional reef standard. Very high PAR output; required for demanding SPS corals before high-quality LEDs existed. Still used in some high-end reef applications for shimmer effect and spectrum. High heat, high electricity cost, and bulb replacement cost limit modern adoption.
Sources: Borneman (2004) Aquarium Corals; Walstad (1999) Ecology of the Planted Aquarium; Riddle coral lighting spectrum research; Giesemann PAR/spectrum studies; Dupla and ADA planted tank lighting guidelines.