not very much data to consider. but a few ideas....... old light bulbs. overfeeding or lack of proper water changes leading to high nitrates / phosphates or just high levels of dissovled organic content.. also the tap water isnt so bad when you talking about a low light tank that evaporates so very little. but on a regular reef tank with med to high light it is deffinatley something you want to avoid. also some tap water may contain higher levels of copper , silica and other undesirables.
but anyway. yep its probably just corals not used to the higher light.
another thing to consider is when going from low to high light, you generaly will be feeding less as the lights will be growing a lot more food( algae, microcrustaceans)