cut off the light during the day for 3 hours. It is OK for a fish but
any
algae should die because of photosintethic problem.

Has this actually worked for you? I do not know anyone who has
succeeded in killing algae by turning off their lights for a couple of
hours midday.

On the other hand, many people has really damaged there plants trying
thing.

- Jim

It wouldn't in any way affect the algae, even if left for more than several
hours...