Why is flash light harmful to what aquatic life forms?
I am sure the main reason is that flash lights, certainly in a dark aquarium, are very frightening and might cause temporary blindness for the animals. If someone flashes in your eyes, you can't see for a few seconds as well. That will cause distress for the animals and distressed animals live considerably shorter.

The total amount of light is indeed much to small to cause algal growth and real blindness is not likely from camera flashes.

EDIT:
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