Direct Sunlight

[Snooks1!]

I've always wondered the exact reason an hour or two of direct sunlight causes algae, but leaving the aquarium light on for 12+ hours doesn't?

Something to do with the dormant algae spores in the water getting the whole spectrum of natural sunlight, thus photosynthesizing faster...Or something like that, right?

 

[Themuleous]

Sounds about right. Algae is far better at utilising sudden changes in the environment than plants are.

Sam