Aquarium Lighting Calculator
The most comprehensive aquarium lighting planning tool available — covering PAR estimation, DLI calculation, photoperiod optimisation, plant mass balance, algae risk scoring, light type comparison, coral acclimation scheduling, and annual running cost in your currency. Supports all tank types from low-tech planted to SPS reef. All seven common light source types. Results in µmol/m²/s, mol/m²/d, lux, and lumens simultaneously.
Aquarium Lighting Calculator
Six modules — use any combination. All results update instantly.
| Light type | Efficiency | PAR quality | Coverage | Spectrum | Heat | Lifespan | For your tank |
|---|
How the Lighting Calculator Works
Aquarium lighting involves several interconnected variables that no single number can capture. PAR tells you how much photosynthetically useful light reaches your plants or corals. DLI tells you the total light dose over a day, combining intensity and duration. Getting both right — and balancing them against your CO₂ supply, nutrient level, and plant or coral demand — determines whether your tank grows and stabilises or generates persistent algae.
PAR — the right metric for plants and corals
PAR (Photosynthetically Active Radiation) measures light in the wavelength range that drives photosynthesis — 400 to 700 nanometres — in units of micromoles of photons per square metre per second. It is the only metric that directly reflects what your plants or corals can actually use. Watts tell you power consumption. Lumens and lux describe how bright light appears to human eyes. Neither tells you anything useful about plant growth. The calculator estimates substrate-level PAR from your fixture's lumen output, depth, water clarity, and light source type — the four variables that most influence how much light actually reaches the bottom of your tank.
DLI — the daily light dose
DLI (Daily Light Integral) is the total photon dose your tank receives over a full day, measured in moles of photons per square metre per day. It combines intensity and duration — you can achieve the same DLI with a bright light running for fewer hours or a dimmer light running for longer. For most planted aquariums the target DLI is between 1.0 and 3.0 mol/m²/day. SPS reef targets are typically 8 to 14 mol/m²/day. The photoperiod module lets you adjust hours to hit a target DLI precisely without changing fixture intensity.
Light intensity vs photoperiod — the balance
This is the most misunderstood relationship in planted aquarium keeping. A higher intensity light running for fewer hours can deliver the same DLI as a lower intensity light running longer — but the two setups are not biologically equivalent. High intensity short photoperiods are generally better for planted tanks because they saturate photosynthesis during the lit period and leave a longer dark period that suppresses algae. Long low-intensity photoperiods produce the same DLI but give algae more hours to exploit the slow trickle of light. The plant balance module scores your specific combination.
Light attenuation through water depth
Water absorbs and scatters light significantly. A light delivering 200 PAR at the water surface may deliver only 80 to 100 PAR at 30 cm depth and 40 to 60 PAR at 50 cm depth in clear water. Tannins, dissolved organics, green water algae, and suspended particles all increase this attenuation. The depth chart in the PAR and DLI module shows estimated PAR at four depths simultaneously, adjusted for your stated water clarity, so you can see exactly what your plants or corals at different heights in the tank are actually receiving.
All Aquarium Light Types — Comparison and Guidance
| Light type | PAR efficiency | Best applications | Lifespan | Heat output | Key consideration |
|---|---|---|---|---|---|
| Full-spectrum planted LED | High — 1.5–3.0 µmol/J | All planted freshwater, shrimp, nano | 30,000–50,000 hrs | Low | Best all-round for planted |
| Reef LED (blue-heavy) | High — 1.5–2.8 µmol/J | All reef types, marine invertebrates | 30,000–50,000 hrs | Low | Best for reef, controllable |
| T5 HO fluorescent | Medium — 0.9–1.4 µmol/J | Reef, planted, Dutch aquascape | 9,000–12,000 hrs (replace at 12 months) | Moderate | Even coverage, quality spectrum |
| T8 fluorescent | Low — 0.5–0.8 µmol/J | Low-tech planted, fish only, display | 7,000–10,000 hrs | Low–moderate | Legacy — adequate for low light only |
| Metal halide (MH) | Low — 0.5–1.0 µmol/J | Deep reef, SPS high PAR, point source shimmer | 6,000–10,000 hrs | Very high | Needs cooling, high running cost |
| MH + LED hybrid | Medium-high | SPS reef — shimmer + controllable spectrum | Mixed — replace MH bulbs | High | SPS specialists' choice |
| T5 + LED hybrid | High | Reef, high-tech planted — even spread + intensity | Mixed — replace T5 tubes | Moderate | Best coverage + controllability |
| Power compact (PC/CFL) | Low-medium — 0.6–0.9 µmol/J | Low-tech planted, soft coral only | 6,000–8,000 hrs | Moderate-high | Largely replaced by LED |
| LED strip (DIY) | Variable — 0.5–2.0 µmol/J | Nano, supplemental, sump refugium | 25,000–40,000 hrs | Low | Quality varies enormously |
| HPS / sodium | Medium — 0.8–1.2 µmol/J | Very large freshwater, rare in hobby | 10,000–20,000 hrs | Very high | Specialist/commercial use only |
Lighting and Algae — Understanding the Connection
Most algae problems in planted aquariums are not caused by having too much light — they are caused by having too much light relative to the tank's ability to use it. Understanding the relationship between light, CO₂, nutrients, and plant mass explains why two identical tanks with identical lights can have entirely different algae outcomes.
Green spot algae
Hard, round, dark green spots on glass and slow-growing leaves. Caused by low phosphate relative to the light level — the tank has enough light to drive algae growth but phosphate has dropped so low that fast-growing plants can no longer outcompete it. The counterintuitive fix is often to increase phosphate dosing rather than reduce light. Green spot algae is a phosphate deficiency indicator in a well-lit tank.
Hair and thread algae
Fine filamentous green or brown algae tangling through plant stems and hardscape. Most commonly caused by unstable CO₂ — fluctuations in CO₂ concentration throughout the day give algae windows to exploit when plants are temporarily carbon-limited. Reducing photoperiod and stabilising CO₂ are the primary fixes, followed by increasing plant mass and fast-growing species to outcompete nutrient availability.
Cyanobacteria (blue-green slime)
A slimy, often foul-smelling mat in red, blue-green, or brown-black colour spreading across substrate and plants. Not true algae — it is photosynthetic bacteria that thrives in low-flow, low-nitrate, high-light conditions. Dead zones in the tank are the primary cause. Increasing flow to affected areas, raising nitrate slightly, and a three-day complete blackout are the most effective interventions. The dead zone planner in the flow and filtration calculator identifies at-risk areas.
Brown diatoms
Soft, dusty brown coating on substrate, glass, and leaves. Almost universal in new tanks and most marine setups — caused by high silicate levels and low light relative to other algae. In new tanks it resolves naturally as silicate is consumed and the biological system establishes. In mature tanks it indicates insufficient light or silicate contamination from substrate or source water. Diatoms are harmless and indicate cycling progress in new setups.
The most reliable first response to any new algae outbreak in a planted tank is to shorten the photoperiod by one to two hours before changing anything else. This reduces the daily light dose without affecting the intensity-driven photosynthesis rate, buying time to identify the underlying cause without risking plant health from a sudden intensity reduction.
Frequently Asked Questions
What PAR level does my aquarium need?
What is DLI and why does it matter more than PAR alone?
How does water depth affect PAR and what can I do about it?
Should I use a siesta lighting schedule?
How do I acclimate new corals to my tank's lighting?
Which is better for reef tanks — LED or T5?
What is the difference between Kelvin rating and PAR?
How often should I replace fluorescent tubes?
Can window light cause algae in my aquarium?
What lighting do jellyfish tanks need?
Complete Your Planted or Reef System Planning
Lighting is one variable in an interconnected system. Once your photoperiod and intensity are set, use the Fertilizer Dosing Calculator to calibrate your nutrient regime to your plant mass and light level — the two must be matched to prevent algae from exploiting excess nutrients that plants cannot use under your current light budget.
For reef systems, use the Water Change Calculator with its parameter planner to track the alkalinity, calcium, and trace element levels that drive coral calcification at the PAR levels this calculator has recommended. The Flow and Filtration Calculator covers the circulation requirements that deliver nutrients to corals and plants receiving your calculated PAR dose.