I'm assuming that 5 gallon tank is 16" x 8" x 10". If so, a horizontally mounted 19 watt powersaver bulb, the spiral screw in type, would give medium high light intensity, and two of those bulbs should give reasonable uniformity of light. 23 watt bulbs would give as much light as you could easily use without loads of algae problems. This is assuming you mount the bulbs in desk lamp style reflectors, the reflectors that are roundish and completely enclose the bulb around all sides. Without reflectors you might only get half to 2/3 that much light.