Changes In Intensity

Plants adapt to a given light source relatively quickly, but a sudden and prolonged significant change in light intensity can be harmful and sometimes deadly. High-intensity lights, such as mercury vapor or metal-halide lamps, may produce too much light for many plants. If the light source is too bright, plants are able to create protective pigments that reduce any harmful effects. However, if a sensitive plant (notably floating species) is transferred from an aquarium with single fluorescent tubes to an aquarium with high-intensity lamps, the plant may not have sufficient time to produce enough protective pigments and will suffer, often dying back quickly.

Aquarium Lamp Plants

Right: The spectral output of a metal-halide lamp shows useful peaks in the blue and yellow areas. The large volume of the curve reflects the high light intensity these lamps produce.

Left: Metal-halide lamps are widely used for marine aquariums, where their spectral output suits marine algae and corals. A lighting canopy with metal-halide lamps and fluorescent tubes will produce a balanced light output for healthy plant growth. The intensity provided by this sort of lighting setup is ideal for deeper planted aquariums.

Right: The spectral output of a metal-halide lamp shows useful peaks in the blue and yellow areas. The large volume of the curve reflects the high light intensity these lamps produce.

Ideal Light Intensity For Human Eye

Right: The spectral output of a white triphosphor fluorescent tube peaks at 400 and 600 nm, ideal for photosynthesis, and at 500 nm, which creates a more balanced light appearance.

Left: Fluorescent tubes are available in different color spectrums for different purposes. Using a combination of tubes in a planted aquarium can create good light for plant growth, as well as creating a pleasing color balance for the human eye.

Right: The spectral output of a white triphosphor fluorescent tube peaks at 400 and 600 nm, ideal for photosynthesis, and at 500 nm, which creates a more balanced light appearance.

Best Spectrum For Aquarium Plants
Above: Although mercury vapor lamps do not produce an ideal spectrum for aquarium plants, their high output easily compensates for this. Mercury vapor lamps also produce an aesthetically attractive light.

provide the most cost-effective and practical solution, as they are cheaper than halogen lamps and provide a much higher output than fluorescent tubes. Common mercury vapor lamps use between 60 and 125 watts, which makes them relatively low-cost to run. If they are within your budget, halogen lamps will provide the best source of light, but mercury vapor lamps are an excellent low-cost alternative.

Other light sources

As well as fluorescent, halogen, and mercury vapor lamps, there are other, less commonly used sources. Sodium lamps and blended spotlamps are two reasonably good sources of light. Sodium lamps are efficient and long-lived although they do lack suitable blue light and may need to be combined with an additional light source. Blended spotlamps are a combination of mercury and tungsten and provide a very well-balanced output at low cost.

Unsuitable lights include any light that does not provide suitable red or blue light and/or provides too low an output. A typical example is the household incandescent (tungsten) bulb, which, although peaking in the red spectrum, is highly inefficient and provides little available light for aquarium plants.

Natural sunlight

At first glance, using natural light to illuminate aquarium plants seems an ideal solution; it is far brighter than fluorescent tubes, has a suitable spectrum, and is cost-free. However, there are disadvantages and generally speaking, you should avoid direct sunlight unless you have plenty of experience and/or are willing to experiment a little. Providing the aquarium is in a place where it can receive sunlight for the majority of the day, all year round, it is possible to create a healthy, well-planted aquarium where fast plant growth will prevent algae from forming. However, this balance is difficult to achieve, and in most cases the plants will either receive too little or too much light. In addition, light levels may vary wildly throughout the year in northern and southern climates or an incorrect balance may cause algal blooms. Sunlight does have advantages in aquariums where light is blocked from overhead, for example from floating plants. In this instance, sunlight through a window will provide lower-growing plants with additional light.

Lighting levels for aquarium plants

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