A one bulb Hagen Glo light might work fairly well for low medium light if you use the "feet" it comes with. Those lights don't have the greatest reflectors, and with just one bulb instead of the usual two, you get less light too. I'm assuming you have a 20 gallon high tank, about 24 inches long and about 16 inches high. If it is a 20 gallon long tank, it is only about 12 inches high, so even with the legs it will be way too much light. To make that work you could use 2 layers of Home Depot fiberglass window screen (insect screen) between the light and the tank, which would drop the intensity down to the low level needed for a "low tech" tank.