by Vaughn Hopkins
We all know that incandescent bulbs are not suitable for lighting a planted aquarium – not enough light and too much heat. But, the “Power Saver” CFL, screw-in fluorescent bulbs, made to replace incandescent bulbs work very well on a planted tank. You just have to use enough of them to cover the whole substrate with light, and use the right wattage bulbs to get the amount of PAR needed for aquatic plants. A typical 10 gallon tank needs two bulbs, for example.
About 2 years ago a member of another forum for planted tanks, i4x4nMore, did some great experiments with CFL bulbs, using a PAR meter to see how well they work. He discovered that there is a very big difference in how much PAR you get when you mount them vertically versus mounting them horizontally. I used his data, and data I got from my own CFL lights to develop a chart that gives approximately how much PAR you can expect for CFL bulbs of various wattages. These are all 6500K color temperature bulbs, which are by far the best to use on a planted tank. The charts are shown below:
Note: If you are not using CO2, try to choose bulbs that will give you a PAR of about 20-30 at the substrate level in the tank.. If you are using CO2, and good fertilizing, choose bulbs for a PAR of about 40-50.
The PAR values shown on the charts are what you should get, using a good reflector for each bulb. You can use ordinary clamp-on work light reflectors from a hardware store or home improvement store for vertical mounted bulbs. Or, there are some commercially available pendant lights, including some made for use on reptile tanks, that also work well and look good. The reflector should be about 8 inches or more in diameter.
If the bulbs are mounted horizontally, in a standard one bulb aquarium T8 light modified to use screw-in bulbs, for example, a reflector can easily be made from thin sheet aluminum, polished enough to look shiny. I found that such a reflector doubles the PAR from the bulbs – the chart assumes such a reflector is used.
Sometimes when you are on a tight budget, using CFL bulbs to light your planted tank is the best available option, and if you find you have too much or too little light with them, you can easily replace the bulbs with different wattage bulbs to adjust the intensity.
This article is part of a series of articles of aquarium plant lighting, and lights for planted tanks in general.