Yup, what ToeJam said.. while ya, there is a slight reduction of clarity the first couple days, it isn't until day 4 that it turns green and gets to the point it looks bad and needs to be cleaned, vs when GFO is spent, that green film appears overnight (clean your glass right before lights go out and the minute the lights come back on the next day the glass is pretty covered in green film), that to me is when I know I need to change the GFO out, and when I do change it out on that day, magic, no more green algae the next morning.. and life is good again. 
