Aquarium Lighting Calculator

Comprehensive Guide: PAR, Spectrum & Photoperiod for Planted Aquariums

Aquarium lighting is both an aesthetic viewing system and the primary engine driving aquatic photosynthesis. In planted aquascapes, lighting intensity must be carefully calibrated against carbon dioxide ($CO_2$) and nutrient availability. Excessive light without proportional $CO_2$ guarantees explosive nuisance algae outbreaks, while insufficient light leads to stem rot, leaf loss, and stunted plant growth.

1. Understanding PAR (Photosynthetically Active Radiation)

In modern planted aquarium science, traditional metrics like "watts per gallon" or "lumens" are obsolete because they measure human visual brightness rather than plant photosynthetic absorption:

• PAR (Photosynthetically Active Radiation): Measures photons in the 400 to 700 nanometer spectrum hitting a given surface area ($mu ext{mol}/ ext{m}^2/ ext{s}$).
• Low-Light Tier (PAR 15–30 at substrate): Ideal for low-demand, slow-growing plants like Anubias, Java Fern, Cryptocoryne, and Java Moss. No pressurized $CO_2$ required.
• Medium-Light Tier (PAR 30–50 at substrate): Suitable for Amazon Swords, Ludwigia repens, Vallisneria, and Bucephalandra. Benefits from moderate fertilization and liquid carbon.
• High-Light Tier (PAR 50–100+ at substrate): Required for dense foreground carpeting species like Monte Carlo, Dwarf Hairgrass, Glossostigma, and red Rotala species. Requires pressurized $CO_2$ injection via our Aquarium CO2 Calculator to prevent algae takeovers.

2. The Inverse-Square Law & Water Depth

Light intensity drops dramatically as it passes through water due to scattering and absorption:

• A light producing PAR 80 at 10 inches depth may produce only PAR 25 at a 20-inch substrate level. Deep aquariums (24+ inches tall) require concentrated optical lenses or higher-wattage fixtures to penetrate to bottom carpeting plants.

3. Color Temperature & Kelvin Spectrum

• 6,500K to 7,500K Daylight Spectrum: Provides the balanced red and blue wavelengths absorbed most efficiently by chlorophyll A and B.
• Full-Spectrum RGB / WRGB: Enhances the vivid red and green coloration of plants and fish scales while maintaining balanced photosynthetic output.

4. Photoperiod Duration & The "Siesta" Schedule

Maintain a consistent 6 to 8 hour photoperiod using an automatic digital timer:

• The Siesta Method: Running lights for 4 hours ON, 2 hours OFF, followed by 4 hours ON allows natural ambient $CO_2$ levels to partially recharge midway through the day, suppressing green spot and hair algae.

Balance your plant layout using our Aquarium Plant Count Calculator. Learn more from Aquatic Plant Central and Tropica Plant Guide. Explore additional tools in our Fish & Aquarium Category.