Try what these guy say. It's the first thing you look at.

From what little I can see it might also be Cyanobacteria. If the normal treatment does not seem to work or makes it worse then you need some Physan20. It's too late for your plants but you will need it to clean up all the nasties.

If it were light leaks you would have a good amount of algea and I don't see it. Looks more like heat and neglect.