If you want to grow plants, stop looking at Lumens and start looking at PAR.
- Lumens are for Humans: Lumens measure how bright a light looks to the human eye. Because our eyes are very sensitive to green light (which plants reflect rather than use), a “bright” light might actually be useless for growth.
- PAR is for Plants: PAR (Photosynthetically Active Radiation) measures the actual intensity of light photons between 400–700nm—the exact range plants use for energy.
- Beam Angle Matters: Standard LEDs have a 120° spread. This is great for wide coverage but poor for depth. Higher-end lights often use 60° or 90° lenses to “laser-focus” the light downward, which is essential for deep tanks.
The Aquarium LED Cheat Sheet
Use this guide to determine if your current light—or the one you’re eyeing—is up to the task.
1. Depth & Intensity Guide
| Tank Depth | Difficulty | Light Type Needed | Plant Recommendations |
| Shallow (6–12″) | Easy | Standard LED strips | Almost anything; easy to carpet hairgrass here. |
| Standard (12–18″) | Moderate | Mid-range LEDs | Stem plants, Amazon Swords, Crypts. |
| Deep (18–24″) | Hard | High-Output LEDs | Focus on taller plants; carpets will need CO2. |
| Extra Deep (24″+) | Expert | Pendants or Spotlights | Hardscape-heavy or very low-light epiphytes. |
2. PAR Targets at the Substrate
To know if your light is “enough,” you need to know what the PAR reading is at the substrate, not at the surface.
- Low Light (15–30 PAR): Anubias, Java Fern, Mosses, Crypts.
- Medium Light (30–80 PAR): Most stem plants, Amazon Swords, Tiger Lotus.
- High Light (80–150+ PAR): Required for red plants and thick carpets (Monte Carlo, Baby Tears). Note: This level almost always requires CO2 to prevent algae.
Deep Tank Strategies
If you have a tank deeper than 24 inches (like a 150-gallon or a tall 55-gallon), you face a unique challenge: the Inverse Square Law. This rule states that if you double the distance from the light, the intensity doesn’t just drop by half—it drops by four times.
How to win with a deep tank:
- Use Pendants: Lights like the Kessil A360X use “Dense Matrix” LEDs and lenses to punch through 30+ inches of water.
- Raise the Plant, Not the Light: Use hardscape (driftwood or tall rocks) to mount your plants higher in the water column. An Anubias attached to the top of a tall piece of wood gets 5x more light than one on the floor.
- Choose “Tall” Species: Jungle Val or tall Aponogetons grow toward the surface, where they can reach the light they need even if the bottom of the tank is dim.
Pro Tip for the “Lazy” Keeper
If you have a deep tank but don’t want to buy a £400 light, just embrace the “Low Light Floor” strategy. Put your light-hungry plants on high driftwood and keep the substrate for low-light Crypts and Mosses that thrive in the shadows.
