I'd take the Tom Barr approach, and say that it's probably not too much nutrients, but too much of the wrong kind, and not enough of the right other stuff. If you have lots of light, and not much carbon, algae will bloom with almost any small amounts of NO3 or NH4. I'd suggest that you do a water change or two, and start adding about 1/4 mL excel every day. Also, add some fast growers as dekstr suggested. I think you'll find that reducing photoperiod won't do much. Let us know how things go.