Quick Answer
Aquarium lighting is the light source and schedule that determines how well fish colors show, how plants photosynthesize, and how much algae growth a tank sustains. For fish-only tanks, any full-spectrum LED on a 6-8 hour timer is sufficient, but planted tanks need lighting matched to PAR output and plant demand, ranging from roughly 20-30 PAR for low-light species to 50-100+ PAR for high-light carpeting plants. The most reliable approach is choosing light intensity based on your plant selection first, then setting photoperiod to 6-8 hours daily on a consistent timer. Running lights longer than 8-10 hours or at intensity beyond what your plants can use is the single most common cause of algae outbreaks in otherwise healthy tanks.
The Problem
I'm not sure what kind of light my tank actually needs, or why my plants aren't growing well while algae keeps taking over instead.
When to Use This Tool
Use this entry when choosing a new aquarium light, troubleshooting poor plant growth, deciding on a lighting schedule, or trying to determine whether your current lighting setup is contributing to algae problems.
Who is this for?
- Aquarium hobbyists setting up new tanks or troubleshooting plant growth and algae issues related to lighting choice or schedule.
Knowledge Journey
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Aquarium Light Types Compared
| Type | Effectiveness for Plants | Cost | Lifespan | Energy Use |
|---|---|---|---|---|
| Full-spectrum LED | High – programmable intensity/spectrum, suitable for low to high-light plants | Moderate-High ($50–300+ depending on size/features) | 30,000–50,000 hours | Low – 30-50% less than fluorescent for equivalent output |
| T5/T8 Fluorescent | Moderate-High – reliable output but fixed spectrum | Low upfront ($20–60), higher long-term (bulb replacement) | 6,000–10,000 hours before replacement needed | Moderate – runs hotter, less efficient than LED |
| Basic LED strip (non-programmable, low PAR) | Low-Moderate – adequate for fish-only or low-light plants only | Low ($15–40) | 20,000–30,000 hours | Low |
| Metal halide | High – very high PAR output, suited to demanding high-light plants | High ($150–400+), plus bulb replacement cost | 10,000–20,000 hours | High – significant heat output, may require cooling fans |
Troubleshooting Guide
1 Added a new plant-rated LED and algae exploded within days.
The new light likely exceeds what your plant mass and CO2/fertilizer levels can use. Reduce photoperiod to 6 hours immediately, dial back intensity if the fixture is dimmable, and only increase again gradually as plant growth and nutrient dosing catch up.
2 Low-light plants are melting or browning even though they're on a low-intensity light.
Check the light is positioned close enough to the plants and hasn't degraded in output — LED diodes lose 10-20 percent intensity over 2-3 years. Also confirm the photoperiod is at least 6 hours; inconsistent or too-short lighting schedules can stress even low-light species.
3 Fish colors look washed out under the new LED compared to the old light.
Many budget LEDs skew toward blue or green spectrum and lack sufficient red/orange wavelengths that intensify fish coloration. Look for a full-spectrum fixture with a color rendering index (CRI) above 90, or add a dedicated red-spectrum supplement channel if your fixture supports it.
4 Carpeting plants are growing tall and leggy instead of low and dense.
This indicates insufficient PAR at the substrate level, common in taller tanks where light intensity drops significantly with depth. Verify actual PAR at substrate height with a meter rather than relying on the fixture's rated output, and raise intensity or lower the fixture closer to the water surface.
Glossary of Terms
- PAR (Photosynthetically Active Radiation)
- A measurement of light intensity usable by plants for photosynthesis, expressed as micromoles per square meter per second, and a more accurate metric than watts or lumens for aquarium lighting.
- Photoperiod
- The number of hours per day aquarium lighting is active; typically 6-8 hours for most freshwater tanks, with longer periods increasing algae risk without added plant benefit.
- Color Rendering Index (CRI)
- A scale from 0-100 measuring how accurately a light source renders colors compared to natural sunlight; higher CRI lighting shows fish and plant colors more vibrantly and naturally.
- Liebig's Law of the Minimum
- The principle that plant growth is limited by whichever resource — light, CO2, or nutrients — is scarcest relative to demand, meaning increasing light alone without matching CO2/nutrients shifts the advantage to algae instead of plants.
Scientific References
- Photosynthetically Active Radiation Requirements Across Aquatic Plant Species
- LED Spectrum Effects on Ornamental Fish Coloration
- Photoperiod Length and Algal Growth Rates in Closed Aquatic Systems