Since light intensity drives the need for everything else, the signs of too much light are wide ranging. Too much light causes a demand for more nutrients than is sometimes being delivered. The list of nutrient deficiency is a long one. Too much light causes a demand for more CO2 than is being delivered. This then causes CO2 deficiency which is characterized by melting, falling leaves, deterioration and rotting, black spots, brown spots, curling or other leaf deformation, as well as holes in leaves. CO2 related algae such as Hair or other Filamentous Algae, BBA or other Red Algae. Diatomic Algae can appear after a lighting “upgrade”. It is entirely possible that the lighting is so high that it requires such high levels of CO2 as to be toxic to the fauna.
Edit; the above was from a CO2 equipment supplier so don't go thinking I'm that smart! However, I do like to do my homework/research before applying changes.