Ideally the guppy should get natural light, since under natural light it appears more naturally colored. The sun’s rays stimulate Vitamin D production and the development of melanin (black pigment) in the guppy’s skin.
The next best choice is fluorescent lighting that has the same color temperature as natural light, called full spectrum. Incandescent lighting is less efficient and generates more heat than fluorescent bulbs. Cheap fluorescent lighting (called “cool light”) is meant for windowless offices and usually casts a greenish pallor on objects and the fish. The only advantage to using such lighting, aside from the low cost, is that you will see your fish under the same lighting as they will be judged in North America. In Europe they will try to judge the fish under natural lighting or full spectrum artificial lighting.
Unfortunately the type of full spectrum light that plants like, algae likes to. Your fish’s natural color will come out in full spectrum light, but if the light is intense, you will be battling algae.
Artificial lighting should supply 0.5 watts of light per liter (or about 2 watts per U.S. gallon). Try to match the “lighting” in the tropics where the guppies come from. Near the equator the day’s length is equal to the night’s (12 hours). Dawn and dusk come and go very fast. However, it’s a good idea to keep a single light on in the fish roomso that the fish are not startled when the main lights come on in the morning. If you have found fish on the floor in the morning, that is probably why.
Put the lights on a timer. Digital timers are more accurate and less fuss than mechanical timers.
A single 40 watt four-foot flourescent tube mounted over your tanks will provide adequate lighting. Many breeders mount the lights in the ceiling. In the tropics pools and streams are often lit by weak light. A rule of thumb is to never have less light than you would use to read a book.
Make sure the lights are plugged into a circuit with a GFI (Ground Fault Interrupter) installed on it. The GFI will shut down the power immediately if there is a short.
What appears to brown algae forming on the side of the tank is usually found to be organic slime. Real brown algae in the aquarium are an indication that the light levels are too low. A persistent problem with green water or severe algae growth on the sides of the tank is usually corrected by reducing organic waste, not changing the light level.