Aquarium Plants

Lighting is the most critical element for a successful planted tank. Plants derive their energy from photosynthesis, without light nothing happens. To make matters complicated algae needs light too, so typically the aquarist is left to figure out how to balance the amount of light in regard to the plant species, nutrients, and CO2 in the tank. Make a mistake with lighting, too much or too little for the plant species, nutrients, or CO2 and you will become a proficient algae farmer...rarely the goal of many people in the hobby. If you've been doing research on the Internet regarding lighting you will have found no other subject as convoluted and contradicted as the subject of lighting a planted tank. I'll try to sort through the terms usually found in describing lighting.

Watts:

Watts is a measure of electrical power that is determined by multiplying the voltage by the amperage. It is not a measurement of light. However is does give a rough idea as to how much light intensity is potentially being directed into the tank. The watts per gallon rule of thumb is based on normal output linear fluorescent bulbs.

Lumens:

This is the tricky term. Lumens is a measure of how much light energy a light source emits. But a lumen measurement is only the energy with wavelengths capable of affecting the human eye. Meaning a bulb with output energy concentrated in the green (550 nm) band would produce more lumens than a bulb with a "full spectrum" pattern...so while it may look brighter to the eye and produce more lumens, it is very likely that plants would "see" the light energy of both bulbs as being the same.

K or Kelvin:

This is an almost laughable measurement. K, K rating, or Kelvin is often referred to as the color temperature of light produced from the bulb. The color temperature of light is derived from the temperature to which you would have to heat a blackbody radiator to make it glow the same color. However, the output from a blackbody radiator is continuous across the visible spectrum but, fluorescent tubes emit an interrupted spectrum, so this means you can't accurately define a fluorescent light as having a Kelvin color, it only approximates it. Bulb manufacturers have stamped bulbs with K ratings to give us an idea of how the color of the light compares to sunlight. Sunlight should be about 5500K. Lower values should look more yellow/orange/red and higher values should look more white/pink/blue. It doesn’t always work out that way, some manufactures use "secret" formulas for determining the K rating they give to their bulbs. A very rough rule of thumb to follow here is any bulb from 2800K to 10000K will grow plants well. Most people, including myself find that bulbs that fall within 5000K to 10000K produce the best appearance and results. Mixing different K rating bulbs is a common choice too. Choose bulbs based on how they render colors of your fish and plants.

Spectrum:

Spectrum is the combination of wavelengths of light that come from the light source. When spectrum is referred to in regards to lighting a planted tank it is usually referencing roughly the six basic colors (and all their hues since the spectrum is continuous) : Red, Orange, Yellow, Green, Blue and Violet. You'll witness many people go to great lengths to claim one bulb has greater peaks in the blue and red parts of their spectral pattern. That's a great thing to have, plants can use red light most efficiently and blue light triggers the plant to take in more CO2...but how do you know if one bulb is really better spectrum-wise. The efficiency of the ballast and how many hours the bulb has been used factor greatly into whether the bulb is still producing the spectral pattern shown on the box. That colorful pattern may be impossible to produce with your ballast or after the bulb has over a 100 hours of use. Just provide enough light and don’t get overly concerned with spectrum.

CRI:

CRI is an acronym for Color Rendering Index. Most people will say it's meaningless. To some degree I'd agree but it does suggest how well a bulb will make colors appear accurate and fully saturated. It's only important for making a decision on how you want your tank to look. Does your bulb washout the colors of your fish? If so you can probably blame it on using a low CRI bulb.

Conclusion

So, in reality, what does all this really mean? Much of the banter on aquaria forums and newsgroups is purely fodder in regards to K ratings and spectral patterns being of any major importance. The attenuation of light in water is wavelength specific, meaning water will absorb light in some bands of the spectrum, organic solutes will absorb blue, phytoplankton absorb blue and red light, and silty or slightly cloudy water absorbs light across all wavelengths. So with this information it should be apparent that plants are exposed to light that is vastly different in quality than simply what's shining down upon them. Moreover, light quality changes rapidly depending on water depth, turbidity, algal blooms and how much organic solutes are in the water. This to me suggests that aquatic plants are flexible and highly adaptable as to their light requirements regarding K-ratings, lumens, CRI, and spectrum and the pursuit of the "perfect" bulb is an act in futility since other factors exert much greater influences on the quality of the light reaching the plant. To further illustrate this point I have personally used "Cool While" bulbs and grew plants with success. However, the color rendering of these "Cool White" bulbs was very unappealing. My point I'm trying to make is this, supply enough intensity for the plants species in your tank and only concern yourself with the minor details of K-rating and CRI for your personally viewing and photographing pleasure. Make it look good and render the colors so that it is pleasing to your eye, the plants will adapt and prosper as long as you provide the intensity. In a low light tank this means 0.5 to 2 watts per gallon of normal output fluorescent lighting and high light is from 3 watts per gallon and up. You'll note that I've got a gap from 2 to 3 watts per gallon. In my experience tanks that fall within that gap are the most difficult to manage. People tend to not use CO2 but try to grow more light demanding plants and balance is more elusive. I'm certain people find success within that range, but personally I've found the two extremes...low light and high light to be much more predictable and the results more repeatable.