Actually, when trying to kill algae a pH of around 7.8 will favor the formation of monochoramine which is extremely effective at killing algae but is very short lived. This is exactly how the ammonia based 'chlorine enhancer' type of algae killers work! If you do this be sure an keep the chlorine levels as constant as you can for best results.
Evan, That's interesting and I have not heard that before. Are you recommending that pH as the most effective level to fight algae?