PROPER LIGHTING:
The beginning aquarist is likely to think that if there's enough light to see then it's enough for plants to grow.
However, that's not true. If you want to grow healthy plants, and not just algae, you need enough light for them to use for photosynthesis and create energy.
It used to be that people advised 3-4 watts per gallon as a VERY basic principle. But, due to modern lighting technology this is now considered an outdated notion (with PAR now more the norm).
The “watts-per-gallon" formula was based on the older T8 & T12 fluorescent lights, many of which were not of optimum PAR/Kelvin and especially PUR, and are severely lacking in the area of lumens per watt. Besides that, "useful light energy" (aka PUR) is something that is often overlooked and is an area where the new generations of LED lights that emit "natural daylight" spectrums cannot be beat.
That being said, along with Metal Halide, the new generation SHO and T5s are also relatively strong in this area. Lumen focus and restrike is an area in which the LED and metal halides reign supreme with almost all light energy directed where the light needs to be: in your aquarium.
The reason this 'watts per gallon' formula can be poor other than comparing LEDs within a specific brand is the efficiencies of so many lights can vary greatly due to design, poor circuitry, low quality emitters, fans, and much more.
With an LED fixture, we can prove our LED efficiency by comparing PAR at the same depth (15" of air is the standard). Using LED fixtures of similar lens angles (120/unlensed to 120, 90 to 90, etc.).
Take the input wattage and divide it by the PAR reading. You will find efficiencies as high as .08 watt of input energy per point of PAR in the AAP Reef White NP 2000 LED to as low as 2.7 watt per point of PAR in the Beamswork EA Timer FSPEC LED. Most of the better LEDs are under .30 watt with most falling in the .4 to .50 watt range such as the Fluval Fresh & Plant 2.0
Reference: Aquarium Lighting; LEDs
Besides watts per gallon these other factors are also quite important:
Lumens per watt.
PAR (often easiest determined by Kelvin output) - this is an area of lighting along with "Useful Energy" where the old "watts-per-gallon" rule/guide really falls apart, especially with modern top tier LED lights.
Lumen focus & restrike
PUR or "Useful Light Energy" (not wasted in yellow/green light spectrum that green plants and zooanthellic algae reflect)
Output in relation to bulb length (this is where LEDs and to a lesser extent T2s and T5s excel).
Lux - I generally only consider this parameter in deeper planted freshwater aquarium to determine if I am getting the proper light where it needs to be.
The starting point of late is PAR, followed by PUR.
Here are some considerations as per PAR (poor spectrums as per PUR would require this to be much higher, such as warm white
* Low light - 15-30 micromols of PAR - CO2 is not needed, but is helpful to the plants
* Medium light - 35-50 micromols of PAR - CO2 may be needed to avoid too many nuisance algae problems
* High light - more than 50 micromols of PAR - pressurized CO2 is essential to avoid major algae problems
Another determining factor is the type of plants you will be keeping.
The common and ACCURATE term would be low, medium, & high light plants. Examples include Java Fern for low light and Wendtii for high light.
Do NOT confuse the term "Low Tech versus High Tech" planted aquarium with "Low Light versus High Light". These two terms are NOT the same!
A Wendtii requires high light regardless of of whether you use a high tech method that involves pressurized CO2, Fert drips or not!!
Based on my many years of experience and 1000s of aquarium set up and kept for clients, you can have an aquarium that produces enough CO2 via "low tech" methods such as employing filters that do not wear off CO2 (such as the Fluidized Sand Bed) along with use of foods and simple supplements that easily can be qualified as low tech, HOWEVER this still does not mean you can get by with a 15 watt T8 cool white fluorescent lamp on a 20 gallon aquarium for a high light plants.
A key point is that light is the controlling factor in planted aquariums. The more light, the more CO2 and Ferts that are required.
This said this correlation does work the somewhat the other way based on my experience and research. Meaning that if you inject too much CO2 & Ferts without increasing light, your growth may not as expected
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