lighting questions


Junior Poster
Jul 13, 2008
Here are the goals for my tank

Twenty gallon light to moderately planted tank.
Low to medium lighting level using compact fluorescents(see below for questions)
DIY co2 at 20ppm
only adding trace elements every few days
Macro's will be provided by food and waste

Lighting questions...

I keep reading about watts per gallon on the internet everywhere. This doesn't make sense to me because you can buy compact fluorescents that are 13W that have the same light output(lumens) as a 60 watt incandescent. Can someone clarify this??? Wouldn't it be more appropriate to say lumens/gallon???

I would like to have a low to medium light level, which i've read is 1-2W / gallon. If i'm using compact fluorescents, what would be the appropriate bulb to use???? Possibly a combination of two???

23W 1600lumens 6500K
23W 1600lumens 5000K
23W 1600lumens 2700K
13W 850lumens 6500K
13W 850lumens 5000K
13W 850lumens 2700K
9W 550lumens 6500K
9W 550lumens 5000K
9W 550lumens 2700K
5W 250lumens 2700K


Lifetime Charter Member
Lifetime Member
Jan 24, 2005
Sacramento, CA
The actual light intensity the plants experience depends on how far they are from the light bulb, what, if anything, is shading them, and how far they are from the center of the bulb, to the side or fore and aft. These variations are not at all trivial. Those bulbs drop in light intensity approximately as the inverse square of the distance from the bulb.

For any kind of bulb, located near the water line, the light intensity at the substrate will be far less than the intensity up just below the water line. That is why a plant can grow so very slowly for weeks right after it is planted, only to explode in growth as it nears the water surface.

All of this is to say there just isn't any way to define the light intensity in an aquarium by the wattage, lumens, or any other parameter available to us for the bulbs. We are stuck with using some parameter that is easily accessible, like watts, and a crude guide like watts per gallon, always with the understanding that such a guide refers to power compact bulbs similar to those used in AH Supply light kits (dual linear tubes), with reflectors comparable to AH Supply reflectors (very good reflectance and shape), and for a conventionally shaped tank, with the lights just above the water.

The screw-in CFL, power saver bulbs are not nearly as good at converting watts to light intensity in an aquarium as the AH Supply type bulbs, mostly because so much light is given off where it can never reach the water, no matter how good the reflector is.

After all of that blather, I suggest you use three of the 23 watt, 6500K bulbs for the 20 gallon tank, hoping that will give you low to medium light intensity. You will want as good a reflector as you can make to have a chance to get that much light. I would also suggest making the light fixture so you can add another bulb if you need to.