You could also get a pleco, though I'm sure you've thought of that. Bristlenose plecos tend to stay pretty small, I've got two of them in my 30 gallon tank and they're both happy, and my tank is clean (and also by a window). Still, I agree with tyspot1000, getting a hood light would be best. Though if you want something cool you could try the 'actinic blue' lights. They're basically black lights for the fish tank, and you can use them exclusively without any harm to the fish.
If you have live plants, you may want to supplement their light from the windows using a bulb within the correct spectrum, that really depends on whether you think they get enough light to thrive or not through the windows.
And if you don't have live plants, then you don't really need any special lighting. But you should still get some sort of hood light, just to help emulate daylight better for the fishies.