Go to your local home improvement or hardware store and buy a fluorescent light fixture which will cover the entire length of the aquarium. For a 50 gallon aquarium this will be a 48” fixture. This will be hung above the aquarium so that it’s no less than 12” above the aquarium itself. This allows the Red Eared Slider Trachemys scripta elegans to absorb the maximum ultraviolet light it needs for the processing of calcium.
The bulbs that give the turtle the UVB they require can be purchased either online or at a local reptile shop. I usually go with the ZooMed® brand as according to the research done at http://www.uvguide.co.uk/index.htm they have the highest output for fluorescent lights. I recommend not using the combination UVB/Heat bulbs which are usually the Mercury Vapor or Metal Halide types of light as these can be easily shattered should water be splashed out onto them when housing an active turtle.
As mentioned above water and lights don’t mix well so I use a ceramic socket reflector for the basking spot and a ceramic heating element of appropriate size. When put into position you are looking for a basking spot of 85 degrees Fahrenheit. Using a ceramic heating element allows you to leave the basking on throughout the evening hours when the fluorescent lights are off. Using any other type of lighting element will disturb the normal circadian rhythm as they will keep the turtle up at night. For more information on heating click here.