Rules, rules, rules, why do we have to follow rules? Well, for the most part, because they are for your own good. Kind of like going to the dentist. However, some rules should not last forever. Sometimes they‘re no longer relevant, sometimes a better idea comes along, and sometimes they turn out to be bad ideas to begin with. This is true in just about any facet of life, and aquarium keeping is no different. I could rant about several rules that come to mind, but my rule of the moment is the old “Watts per gallon” rule when it comes to choosing lights for your aquarium, particularly live plant and reef aquariums, which require higher intensity lighting.
Watts are a measure of energy consumed by a lamp, not the light energy that is produced. Using a rule based on watts is flawed from the get go, because it doesn’t consider all of the aspects that contribute to light output of bulb. I understand where the problem comes from. Back in the day, when aquarium technology dinosaurs roamed, there were few options for lighting your aquarium. For the most part people were limited to standard fluorescent tubes, all of a similar lumen per watt output, so there was a direct relationship for watts and lumen output for most lighting options. That could not be further from the truth when it comes to modern lighting systems available to today’s consumers. Metal halide, T12, T8, T5 and LED bulbs are wildly different in their efficiency, and they make any sort of rule based purely on watts obsolete. All watts are not created equal.
The lumen (symbol: lm) is the SI unit of luminous flux, a measure of the power of light perceived by the human eye. The efficiency of a light bulb can then be looked at as Lumens produced per watt of energy consumed. When looking at lumens per watt, the different bulbs types available today show just how difficult a question this can become. To further complicate things, you must look at the requirements for photosynthesis, in respect to the spectrum of light. Is the light being produced usable? PAR (Photosythetically Available Radiation) is the portion of sunlight that is used in photosynthesis, and has wavelength between 400 nm and 700 nm. It is possible to have a high wattage bulb, with low PAR values. Modern bulbs are much more efficient than older technology. HO T5 and LED bulbs are much more efficient that PC and Metal Halide bulbs, and the LED’s in particular produce much less heat to deal with.
If you are not confused yet, there has still been no consideration for water depth, which plays a huge role how much light can penetrate to the bottom of an aquarium. Tanks of similar volume, can have very different dimensions. Deeper tanks need much more light energy than shallow tanks for keeping live plants or corals happy at the bottom of the tank. Light energy is quickly absorbed by water, with only blue wavelengths of light being able to penetrate deeply into water.
So how does it all relate to your aquarium? Nowadays, when considering what lights to use on your aquarium, you must consider bulb efficiency (lumens per watt), light quality for photosynthesis (PAR), and tank depth to make an informed decision, depending on what you’re trying to keep. Unfortunately, most of these things are not readily available on packaging and it can take a little work to calculate which lights will suit your plants or corals requirements.
So the old watts per gallon rule, it needs to go. Every situation is different, and no one rule can apply to all tanks. Not all watts are created equal.