As we said, there’s no universal, “magic amount” of light we can share with you–but we do have the next best thing: General rules of thumb.
Lumens per Liter
For example, the plants in your aquarium will typically require at least 10 lumens per liter. If it’s a light-hungry plant, it could require closer to 40 lumens (which is a full watt) per liter. Medium-light plants would fall in the 20 to 40 range.
This isn’t perfect. The light won’t spread uniformly across every liter of your tank. The deeper the light has to travel, the weaker it gets.
Even though lumens per liter is imperfect, it can still serve as a helpful starting point for gauging your plants’ needs and the strength of your light relative to your tank.
Plants only use visible light for photosynthesis. Photo Credit Dennis Wong
Watts per Gallon
A metric that isn’t quite as helpful for LEDs is WPG or Watts per Gallon.
There’s some controversy around WPG:
In the blue corner, you have aquarists arguing that it can give you a “rough idea” of how much light your tank has/needs, and it’s not any less valuable than a PAR reading.
In the red corner, you have aquarists arguing that it was a method devised years ago that was never quite used correctly, and now it makes no sense–especially when it comes to LED lights.
LED lights give off similar levels of light as T5 or T8 bulb lights, but with a lot less wattage. If you tried to match old T5 watts per gallon numbers with energy-efficient LED lights of today, you’d likely kill your plants and grow some serious algae.
And, as we said earlier, the light in a tank is not uniform. It’s not like you’re not mixing cocoa powder into a cup of hot milk–1 watt per gallon doesn’t actually mean 1 watt is mixed in with each gallon of water.
But even as we sit here debating the validity of old-school guidelines, many planted tank hobbyists (both old and new) are successfully using WPG as a way to loosely guide their planted tank lighting setups.
Photo Credit: Dennis Wong
PAR is another light measuring system that isn’t without controversy. PAR stands for photosynthetic active radiation. It refers to the amount of light available for plants to synthesize food.
A PAR meter can tell you how light much could be used, but not where in the spectrum the light falls. Plus, PAR meters can run you up about $350. If you’re a DIY champ, you can rig one up for closer to $100.
Since PAR doesn’t measure the spectrum of light, the PAR reading isn’t perfect. Plants absorb certain wavelengths of light more readily than others.
What does it mean?
No method is perfect, but tens of thousands of aquarists are still able to grow plants in their aquariums successfully.