This is an inexpensive way to light a fish tank using screw type compact fluorescent bulbs. You will need just 7 very inexpensive items to complete this project. Here is an itemized list.
Items Needed:
2 x Socket Adapters ($1.79 each)
2 x Compact fluorescent (26w or higher) screw in type bulbs with built in ballasts. ($7.98 for a 3 pack)
1 x Extension cord ($1.09)
Aluminum Foil ($1.69) (optional)
Silicone Sealant ($3.98)
Screws
Wire to hang the fixture with
Total Cost: $18.31
Directions:
Step 1: Screw the CF light bulbs into the socket adapters.

Step 2: Plug the 2 socket adapters into opposite sides of the extension cord.
Step 3: Seal the connection between the adapters and the cord as well as the bulb and the adapter with silicone sealant to make it water resistant.

Step 4: If the inside of the hood is not reflective, you can use the silicone sealant or other adhesive to glue aluminum foil to the inside of the hood.
Step 5: Attach the fixture to your hood using screws and wire (or choose your own method depending on your specific installation)
Step 6: You can also put an inline switch on the extension cord to make it easy to turn off and on.
Here is the completed project:

DIY Compact Fluorescent Lighting For Less Than 20 Bucks!