In a south facing window I don't think sunlight would be a problem. South facing windows would have it the worst. The idea of wanting algae in your tank sound very interesting. It makes me want to get a 55 gallon, stock it with bristlenose plecos and true Siamese algae eaters, put it in a south facing window and watch what happens. For your tank I'd keep it low light, a lot of people want show quality tanks but that requires a lot of nutrients and co2 injection. My first true planted tank(a 20 gallon) had java fern, amazon sword, anacharis, and aponegetons all with just two 25 watt cfls in the stock fixture. No ferts, no co2 and the best looking java ferns to date. Keep it simple. Good luck.