While mollies do need some specialized aquarium care, they usually don't require specialized light. However, mollies do benefit from having live plants in their aquarium, and live plants need intense lighting -- and other care -- to thrive. So while mollies themselves might not need powerful lighting, their ideal aquarium might.
How Much Lighting
In a fish-only aquarium, you only need enough light to see your mollies. Mollies do not require light directly. For a fish-only aquarium featuring mollies, 1 watt of lighting power per gallon of aquarium volume is usually plenty to see your fish. For tanks larger than 50 gallons, you should probably up this to 2 watts per gallon to accommodated the greater depth. However, planted aquariums need more light, anywhere from 1.5 to 5 watts per gallon, depending on what kind of plants you wish to keep. As tropical fish, you should give them a photoperiod -- the amount of time the lights are on per day -- of 8 to 12 hours. You can use an inexpensive lighting timer from the hardware store to give the fish consistency.
The aquarium hobby features a variety of fluorescent lighting, ranging from basic aquarium lights to sophisticated systems capable of sustaining coral. If you only want to keep mollies by themselves, a fluorescent light makes a good, cheap light. If you want to keep plants, you can purchase special fluorescent tubes designed for promoting plant growth. With planted tanks, you should swap out your bulbs every six months, since they subtly fade and their spectrum shifts as they age. These changes happen long before the bulb actually burns out, but the changes matter to plant life.
Saltwater hobbyists use metal halide bulbs more often than their freshwater counterparts. However, aquarium plants in a molly tank could benefit from metal halide bulbs. These lights produce powerful illumination that easily penetrates to the bottom of deep aquariums, making them ideal for large reef tanks. However, freshwater aquarium plants, a favorite food of the molly, will also respond well to these bulbs. Keep in mind, they cost more than all but the most advanced fluorescent fixture and produce a lot of excess heat. However, if you keep your mollies in a deep, planted aquarium, these lights could be ideal.
LED arrays are the latest innovation in aquarium lighting, and they work well for both mollies and their favorite aquarium plants. Right now, the initial cost to buy them is high, but they are cheaper to operate than other types of lighting. LEDs use about 20 percent of the power to produce the same amount of light as metal halides. Additionally, LEDs last longer than any kind of bulb. LEDs are currently estimated to work for at least a decade. On top of these benefits, the cost of store-bought LED arrays is dropping as more aquarium hobbyist adopt the technology.