If that is so, you have killed a lot of algae, I would do a big water change.
That much matter dying all at once can add too much nitrogen to the tank, the ammonia and nitrite removing bacteria cannot keep up.
Fish piping: I would do a water change.
If the posts above are right, and your tap water comes pre-loaded with algae, I would not dose the high initial dose of Excel. Just do the regular dosing.
Water change? How big of a water change?