T5 lighting would be nice. Cheap, energy efficient and works well for 20 gallons (ASSuming yours is a 20 long and not a 20 high or xtra high).
http://www.bigalsonline.com/BigAlsUS...lestriplight30
1 fixture will give you more lumens to allow you to keep lower light plants and some lights that require a little more light. 2 Fixtures would allow you to keep all but the most demanding lights. T5 lights are way more efficient than standard flourescent bulbs so dont worry about the wpg.
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