If you don't have aquatic plants, and have a dense growth of terrestrial plants above the water, you can use a lot of light without too much problem with algae underwater. Will there be fish in the tank?
I assume the tank is 36 inches long, so you will want a 36 inch long light. You could use a hydroponic T5HO light, which is cheaper than aquarium lights. I don't think you will want more than a one bulb light though.