Lighting Explained: PAR, Spectrum, Photoperiod for Planted Tanks
1) PAR in the Real World: Depth, Hardscape Shadows & Plant Demand
PAR (Photosynthetically Active Radiation) measures the photons in the 400–700 nm range that plants can use. In practice, aquarists use PPFD (photosynthetic photon flux density) in µmol·m−2·s−1, which tells you how much plant‑usable light actually reaches a point in your tank. Numbers on a box rarely translate directly into in‑tank results because water depth, glass covers, surface agitation, and hardscape shadows reshape your light field dramatically.
Depth & attenuation. Light intensity falls with depth due to scattering and absorption. In shallow nano tanks (20–30 cm water column), even modest fixtures can deliver medium‑high PAR to the substrate. In deeper tanks (45–60+ cm), PAR at the carpet might be a fraction of the surface value. Blackwater tint and tannins further reduce PAR—often beneficial for soft‑water fish but a constraint for demanding carpets. This is why two keepers can run the same fixture and get wildly different outcomes.
Measuring what matters. A handheld PAR meter is ideal, but you can also infer effective PAR by plant response and algae behavior. Compact carpets (Monte Carlo, dwarf hairgrass) that hug the substrate and produce runners typically indicate at least low‑to‑medium PPFD at the bed (≈30–60). Red stem plants with dense internodes and crisp coloration usually point to stable high PPFD and CO₂. Persistent green spot algae on glass hints at too much light relative to phosphate or CO₂ delivery.
Plant demand tiers. A practical framework: low light ≈ 20–40 µmol for shade‑tolerant species (Anubias, ferns, many crypts), medium ≈ 40–70 for robust stems and easy carpets, and high ≈ 70–120+ for colorful stems and picky carpets. These bands assume reliable CO₂ at the leaf; without CO₂, shift your expectations down and be conservative with PAR. If your PAR is high but CO₂ is weak, you’re just feeding algae with photons.
Distribution & shadows. Rock spires, wood, and tall stems create micro‑climates. Use two smaller fixtures spaced apart or a wide‑angle unit to reduce hot spots and shaded holes. Angle the light or raise it a few centimeters to broaden the footprint; a small height change can even out PAR more effectively than cranking intensity. Keep glass lids clean—salt creep and condensation can easily sap 10–20% of delivered light.
Actionable takeaway. Don’t chase a printed wattage. Decide your plant goals, then set a target PAR band at the substrate and test/observe. Start lower than you think, let plants adapt, and increase in small steps only after CO₂ and nutrients are demonstrably steady.
2) Spectrum Demystified: What Colors Plants Actually Use
Spectrum is the distribution of wavelengths your light emits. Aquatic plants use a broad span of PAR, with blue (≈450 nm) driving compact growth and red (≈660 nm) fueling photosynthesis efficiency; green penetrates deeper into canopies and water, improving overall light use. The myth that “plants don’t use green” is outdated—leaves reflect a lot of green, but enough is absorbed to matter, especially in dense scapes.
White LEDs vs RGB. Most aquarium lights use white LEDs (blue diodes with phosphor) balanced to 5000–8000 K; others add separate red/green/blue channels for tunability. The good news: healthy plants are more sensitive to total photons and CO₂ stability than to fine spectral tweaks. That said, spectrum influences color rendition (how your scape looks) and can gently nudge plant morphology.
Color temperature and rendering. A 6500 K “daylight” look is neutral and flattering for greens. Slightly warmer mixes bring out reds and browns, while cooler mixes emphasize a crisp, modern aesthetic. If your fixture allows tuning, keep changes small and observe for two weeks—dramatic shifts can upset algae balance by altering how light penetrates and how plants acclimate.
Reds & anthocyanin. Vivid red stems aren’t just spectrum; they require sufficient PPFD, stable CO₂, and consistent iron/micros. Additional deep red (≈660 nm) can help when the other fundamentals are in place, but it won’t rescue weak CO₂. Think of red channels as “polish” after you’ve nailed PAR and gas.
Fish and viewing comfort. Spectrum also affects livestock perception and stress. Avoid harsh blue spikes at night—fish need darkness to rest. For tannin‑rich, blackwater setups, a slightly warmer spectrum harmonizes with the amber hue and reduces glare. If you keep bettas or labyrinth fish, provide floating cover that diffuses point‑source glare; they often hang near the surface where lights are brightest.
Actionable takeaway. Choose a reputable fixture with even coverage and sufficient output. Use spectrum to taste—for color and mood—once growth is steady. Keep a “reference preset” saved so you can revert if a tweak triggers nuisance algae.
3) Photoperiod Tuning: How Long, Split Schedules & Algae Control
Photoperiod is how long you run the lights each day. In closed systems, plants saturate at surprisingly modest durations once PAR and CO₂ are adequate; beyond that, extra hours mostly feed algae and stress fish. Smart photoperiods balance growth with stability and your daily routine.
Start conservative. For new or recently reset tanks, run 6 hours/day for the first 2–3 weeks while biofilms establish. Increase to 7–8 hours once you see steady plant pearling and minimal algae. Mature, well‑balanced high‑tech scapes rarely need more than 8–9 hours; low‑tech tanks often thrive at 6–7 hours to keep algae pressure low.
Split schedules. A “siesta” (e.g., 4 hours on, 2 hours off, 4 hours on) can help some tanks: it lets dissolved CO₂ rebound and gives fish a mid‑day rest. Use this if your CO₂ system struggles to maintain levels through the full day or if room sunlight adds a noon spike that you want to bypass. Keep total light time the same; splitting isn’t an excuse to run longer overall.
CO₂ ramping and on/off choreography. If you inject CO₂, start gas 1–2 hours before lights so concentration is at target when PAR rises. Turn CO₂ off 1 hour before lights out. Gentle ramp‑up/ramp‑down on dimmable fixtures reduces stress for fish and reduces sudden pH swings. Tie your photoperiod to feeding times so you can observe behavior under good light without overextending hours.
Algae pressure & diagnostics. Brown diatoms in young tanks fade with time and silica consumption; green dust algae on glass often clears after a full life cycle if you resist scraping too often; persistent green spot suggests either too much light for your phosphate or inadequate flow over leaf surfaces. When algae pushes back, it’s usually more productive to shorten the photoperiod by 30–60 minutes and improve CO₂ distribution than to crank fertilizers blindly.
Actionable takeaway. Treat hours like fertilizer dose—adjust slowly, one change at a time, and give the system two weeks to respond. Keep a simple log of “hours / CO₂ timing / algae notes” to find your tank’s sweet spot.
4) Putting It Together: Matching Light to CO₂, Nutrients & Goals
Light, CO₂, and nutrients form a triangle. Push one corner and you must support the others or the system tilts. Many algae problems aren’t “too much light” in absolute terms—they’re too much light for the available CO₂ at the leaf or for the nutrient/flow regime.
Match intensity to gas. If your drop checker never reaches lime green or if pH drop (relative to degassed) is less than ≈1.0, treat your tank as medium light no matter what the fixture can do. Raise the light or dial it down until CO₂ hits target reliably. Even coverage matters more than raw PAR—dead spots invite algae on leaves even when average PAR looks fine.
Nutrients & dosing frameworks. For high‑tech scapes, consistent macro/micro dosing (e.g., EI‑style) paired with weekly 30–50% water changes keeps nutrients non‑limiting and predictable. For low‑tech, dose lightly and focus on root tabs under heavy feeders. Either way, keep the mechanical filter stage clean so dissolved organics don’t soak up light and fuel film algae.
Flow & oxygenation. Strong surface ripple improves gas exchange—vital at night when plants respire and during heat waves. Angle spray bars to sweep plant beds; add a small powerhead behind hardscape to eliminate detritus pockets. Healthy, moving water lets plants use the photons you’re paying for.
Practical build recipes. (1) Low‑tech jungle: wide‑angle LED at modest intensity, 6–7 hours, no CO₂, easy plants; change water weekly and keep filters clean. (2) Nature style with CO₂: even coverage LED(s) delivering substrate PPFD ≈ 60–80, 7–8 hours, stable 1.0 pH drop, EI or lean dosing with weekly resets. (3) Iwagumi show tank: high, evenly distributed PAR (80–120 at carpet), 7–8 hours, immaculate CO₂ distribution and nutrient discipline; trim frequently to keep shadows in check.
Actionable takeaway. Treat light as the accelerator. Press it only as far as your CO₂ and maintenance can steer. When in doubt, reduce intensity or hours first; growth slows, but stability returns—and stability is what creates the best-looking aquascapes.
FAQ
Do I need a PAR meter?
Helpful but not mandatory. Many successful scapers tune by plant response and algae cues. If budget allows, a meter accelerates learning and removes guesswork.
What color temperature is best?
Around 6500 K is a safe, natural baseline. Adjust warmer/cooler for aesthetics once growth is stable.
How long should I run lights in a new tank?
Start with 6 hours for 2–3 weeks, then step up to 7–8 hours as plants establish and algae pressure decreases.
Next reads: CO₂ Systems: DIY vs Pressurized • Low-Tech Planted Tanks • EI Fertilization • CO₂ Tuning with pH Drop • Aquascape Photography
Labels: Lighting, PAR, Spectrum, Photoperiod, Planted Tanks, Beginner Guide