Ratio of lumens per liter in the planted aquarium

In the aquarium hobby, the Watts/liters (W/L) ratio was traditionally used as a reference to talk about the “amount of light” necessary for the aquarium. For example, it was said of an aquarium of 80 liters and with 40 Watts of lighting, that it had 2L/1W or 0.5W/1L.

But nowadays it is much more appropriate to talk about lumens per liter, which is what we are going to see next. This article is part of our guide on aquarium lighting, where we cover all things light in the aquarium.

Lumens / Liters ratio (Lm/L)

When we are talking about lighting for fish tanks, we are more interested in knowing the «amount of light» that an aquarium lamp emits, than the power it uses. Although this also matters, because the higher the power, the higher the electricity consumption (+ little money).

To measure the amount of light emitted by a luminaire, the Lumen (Lm) is used. Technically it measures the luminous flux:

Amount of light energy per unit of time.

To finish clarifying these basic concepts about aquarium lights, I will give you an example:

A high quality luminaire may only need 50W to emit more Lumens (more amount of light) than a low quality 100W luminaire.

It is the performance of the aquarium lamp. The relationship between the power and the luminous flux of the luminaire.

We continue with the example from before:

Imagine that the 50W fish tank lamp emits 3,000Lm, so it has a performance of 60 Lm/W.

The other emits 4,500Lm with its 100W, obtaining a lower performance of 45 Lm/W.

Okay, now… But how much light is necessary for my aquarium? How many lumens does a planted aquarium need?

Based on this, here is another generic aquarium lighting list, but expressed in a Lumen/Liter (Lm/L) ratio:

  • 20-25 Lm/l – Basic fish tank light in freshwater tanks without plants.
  • 25-35 Lm/l – intermediate aquarium light for sweet fish and plant tanks.
  • +35 Lm/l – strong aquarium lighting for planted aquarium.
  • 50 Lm/l – marine aquarium lights.
  • 50-70 Lm/l – marine reef fish tank lights.

Please note that it is just a reference. The rest of the variables must be taken into account, such as the depth of the tank. Light does not spread evenly through every liter of tank. The deeper the light has to travel, the weaker it becomes.

Although lumens per liter is not perfect, it does serve as a starting point for gauging the needs of your plants and the amount of light in relation to your tank.

Because of this, most reputable manufacturers provide lumen ratings so you know where each model can be best used.

In any case, when your aquarium has been around for a few months, you will only have to look at the color of the algae because it will tell you what the light in your aquarium is like:

  • Brown algae on sand or glass indicates weak or poor quality fish tank lights.

  • Green color in algae means correct aquarium light.

  • Blue algae indicate excess light intensity from your fish tank lights.

Watts / Liters (W/L) Ratio

For a long time, when talking about aquarium light, this ratio of aquarium light and capacity in W/L has been used.

The main error with this reference is that the watts speak of the power of electrical energy that the lamp needs/consumes. It is true that in general, the more a luminaire consumes, the more light it produces. But the reality is that with the W we do not talk at any time about the quality or quantity of light.

When we talk about Watts, we are talking about the energy power that the aquarium lights need to be able to work.

Therefore, this reference is not very successful, and even less so with LED technology. Why? Why LED lights use much less power to emit a similar level of light as T5 or T8 fluorescents.

It’s not a good idea to try to match the watt/liter ratio of the old T5’s with today’s energy efficient planted aquarium LED displays. You will most likely kill your plants and need welding goggles to look at your aquarium.

Before, these general references were used in fish tank lights, now obsolete with fluorescent aquarium lamps in freshwater tanks:

  • 4L/1W [0.25W/1L] – basic aquarium light.
  • 3L/1W [0.33W/1L] – intermediate fish tank light.
  • 2L/1W [0.50W/1L] – strong aquarium light.

For marine aquariums, a 2L/1W ratio was established.

Currently it is more correct to use the Lumens / Liters (Lm/L) ratio because the Lumen does measure the «amount of light». The Lumen measures the luminous flux, that is, it tells us about the amount of light, which interests us much more than the power of the lamp.

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