Why does my light "look bright" but the plants are not growing?

Visible brightness (lumens) and photosynthetically active radiation (PAR) are different measurements. Human eyes are most sensitive to green-yellow light around 550 nm; plants use red and blue wavelengths for photosynthesis. A fixture can look dazzling to the eye while producing mediocre PAR if the spectrum is tuned for appearance rather than plant response. The calculator assumes planted-spectrum fixtures; aquarium moonlights, white-only LEDs, and generic office-lighting units often underperform their lumen rating by half in PAR terms.

How do I reduce algae without cutting the photoperiod drastically?

Algae thrives when light energy exceeds what plants can consume. The reliable fixes, in order of impact: add or increase CO₂ (instantly raises plant growth rate), add more fast-growing stem plants (consume the surplus nutrients and light), tune the fertiliser routine so plants are not nutrient-limited, dim the fixture or raise it, then cut photoperiod. Going below 6 hours of light starts limiting plants before it limits algae, so photoperiod cuts below 7 hours are a last resort rather than a first fix.

Does a midday break in the photoperiod actually help?

Siesta schedules (4 hours on, 3 hours off, 4 hours on) became popular around 2010 on the theory that algae needs continuous light to spore while plants recover instantly. Practical research by Tom Barr and others has found the effect is small to non-existent in well-run tanks — algae is better correlated with nutrient imbalance and CO₂ stability than photoperiod interruption. A continuous 7-8 hour photoperiod is simpler and produces equivalent or better results than a siesta in most planted tanks.

What about "low light" anubias and java fern on a high-end LED?

High PAR on low-demand plants typically does not harm the plants but creates algae problems on the plant leaves themselves. Anubias and java fern grow slowly, so even moderate PAR outpaces their consumption and leaves surplus energy on the slow-growing leaf surface — prime algae real estate. Either shade them under taller stems, dim the fixture, or switch to a planting that matches your PAR budget. Pair lighting decisions with the stocking plan — a few fast grazers (amano shrimp, otocinclus) help control leaf algae that lighting alone cannot prevent.

Aquarium Lighting Calculator — PAR Guide

The Aquarium Lighting Calculator estimates PAR at substrate, recommends a photoperiod, and flags algae risk for planted freshwater aquariums based on tank depth, light type, plant demand, and whether CO₂ is present.

Why Light Is the Master Variable

Light drives everything in a planted tank — get it wrong and you grow algae instead of plants. The photosynthesis reaction needs three inputs: light energy (PAR), carbon (CO₂), and mineral nutrients (fertiliser). Plants are efficient at this reaction when all three are balanced; algae is a backup opportunist that consumes whichever resource plants leave unused. Too much PAR relative to available CO₂ and nutrients means the surplus energy feeds algae. Too little PAR means plants cannot photosynthesise fast enough to outcompete the algae that always-exist in every tank. The goal is dialling PAR to match what your plant mass can consume, then supplying CO₂ and nutrients to keep up.

The measurement that matters is Photosynthetically Active Radiation (PAR) — the energy in the 400-700 nm wavelength band that plants use for photosynthesis, expressed as micromoles of photons per square metre per second (µmol). Plant demand sorts into rough bands: low-demand anubias, java fern, and mosses need 15-30 µmol; medium-demand cryptocoryne and amazon swords want 30-50; high-demand stems like rotala and ludwigia need 50-100; carpet plants (Monte Carlo, HC, dwarf hairgrass) want 100+.

Depth Attenuates PAR — Fast

The number quoted on manufacturer PAR charts is usually measured at 30 cm below the fixture, which matches the typical water-surface distance on a home aquarium. But most plants sit at the substrate, not the surface, and PAR decreases as it travels through water — roughly 50% loss per 30 cm for clean aquarium water. This is the hidden constraint most beginners miss.

A standard planted LED producing 60 µmol at the water surface delivers only 30 µmol at 30 cm depth, 21 µmol at 45 cm, and about 15 µmol at 60 cm. The same fixture that grows a carpet in a shallow 30 cm aquascape cannot support even medium-demand plants in a 50 cm tall show tank. High-end LED fixtures (Chihiros WRGB, Twinstar, ADA Aquasky) double this number — roughly 120 µmol at surface, 60 µmol at 30 cm, 42 µmol at 45 cm, 30 µmol at 60 cm. Deep tanks almost always need high-end fixtures or pendant setups to reach carpet-demanding PAR; standard LEDs max out on medium-demand stems in tanks under 45 cm.

The Algae Risk Triangle

Algae is never caused by "too much light" in isolation. It is caused by light energy exceeding what plants can consume. This happens when any leg of the light-CO₂-nutrient triangle is out of balance. The classic algae triggers are: high PAR + no CO₂ injection (plants cannot photosynthesise fast enough to use the light), high PAR + sparse plants (not enough plant mass to consume the energy regardless of CO₂), and any PAR level with photoperiods stretching past 10 hours (extended light periods favour algae spore development over plant recovery).

The calculator runs this triangle for you. If PAR is above 50 µmol at substrate, CO₂ is off, and plant demand is low or medium, it flags HIGH algae risk — this combination produces algae within weeks regardless of other variables. If PAR is above 70 µmol without CO₂ under any plant selection, the flag is HIGH — high-tech PAR levels require high-tech support to function without algae. Medium risk covers the boundary cases where the setup could work with careful nutrient management but is not a forgiving configuration.

Photoperiod — Simpler Than Most Guides Claim

Photoperiod (daily light hours) is the second lever after PAR. The workable range is 6-10 hours depending on PAR level. High PAR setups (>70 µmol at substrate) should run 6-8 hours — shorter photoperiods suppress the extended growth phase where algae catches up. Medium PAR runs 8-9 hours as the standard. Low PAR can stretch to 9-10 hours, but not longer — photoperiods above 10 hours consistently favour algae over plants regardless of PAR, an effect independent of light energy that relates to algae spore development cycles.

Siesta schedules (split photoperiods with a midday break) became popular around 2010 on the theory that the interruption disrupts algae spores while plants recover instantly. Practical research has found the effect is small to non-existent in well-run tanks — a continuous 7-8 hour photoperiod produces equivalent results with less complexity. The one situation where a siesta helps is a tank that receives strong indirect daylight during the day; splitting the artificial photoperiod around the daylight peak avoids compound high-PAR periods.

New Tank Ramp-Up

A brand-new planted tank should not start at target photoperiod. Algae always establishes faster than plants in the first 2-3 weeks because plants are still rooting and transitioning from emersed (air) to submerged growth. Start at 6 hours for the first two weeks, then increase by 30 minutes every few days until reaching the target. This gives plants time to build biomass before peak light energy arrives. Running full photoperiod on day one in a fresh tank is the classic setup mistake that produces the green-hair-algae outbreak every aquascaper remembers from their first tank.

If algae appears during ramp-up, slow the ramp rather than reversing it. Algae on new plant leaves is normal and transient; the plants will outgrow it as they establish. Panicking and cutting the photoperiod back to 5 hours prolongs the establishment phase and makes the problem worse. Pair the ramp-up with a disciplined maintenance schedule — weekly 30-50% water changes clear excess nutrients that algae uses in the establishment phase.

Fixture Category, Not Brand

Brand names change every few years — the fixture you buy in 2026 will be replaced by a newer model in 2028, and preset recommendations by model name go stale fast. Think in fixture categories instead. High-end planted LEDs deliver around 120 µmol at 30 cm and include adjustable spectrum, sunrise/sunset ramping, and app control (Chihiros WRGB II, Twinstar, ADA Aquasky, Fluval Plant Spec). Standard planted LEDs deliver around 60 µmol with basic timer support (Fluval Plant 3.0, Nicrew ClassicLED Plus, Finnex Planted+). T5HO fluorescent fixtures deliver around 80 µmol with dual-tube setups — older but still viable and often cheaper to replace tubes than buy new LEDs. T8 fluorescent is older single-tube technology around 30 µmol — inadequate for most modern planted tanks past 40 cm depth.

Within each category, prices vary 3-4× but PAR varies much less. A £150 high-end LED and a £400 high-end LED produce similar PAR in the planted bands; the premium tier buys build quality, longer warranty, and spectrum customisation rather than raw output. For most hobbyists, the mid-tier product in the right category is the best value.

What to Actually Do

The calculator returns an intensity guidance line based on whether PAR is under, in-range, or over target. Under-target on T8 flags upgrade as the solution. Under-target with a suitable fixture category flags fixture height adjustment (lowering closer to surface by 5-10 cm) and photoperiod extension as cheaper interim fixes. Over-target flags dimming, raising the fixture, or cutting photoperiod. In-range flags maintaining the setup and adjusting only if algae appears.

Light is one variable in a multi-variable system. A balanced CO₂ injection tuning and fertiliser routine matter as much as PAR — dialling light alone while leaving CO₂ and nutrients unmatched just moves the imbalance around. Plan all three together, and the substrate depth planning and stocking plan feed back into the same tank balance. Regular tank upkeep through the maintenance schedule keeps nutrient levels from drifting in ways that break lighting decisions made months earlier. The discipline parallels matching inputs to goals in canine exercise — the balancing idea surfaces in other pet contexts, and the similar matching discipline in breed-specific exercise covers the theme from a different angle.

Aquarium Lighting Calculator