He's talking about the 35% H2O2 to be used at 1ml/gallon. For drugstore H2O2, which is 3%, you would use 15-20 ml/gallon. A tablespoon is 15 ml.
But the BEST algae killer is darkness. The algae is growing because your light is hitting a wet surface, so cover that area and no algae will grow. But you'll still have to kill the algae that is growing now first.
HTH
