What about after adding fish? The water would have been fine before the fish went in as there was nothing in there to make it 'un-fine'.
Fish excrete ammonia, it's their version of urine. This ammonia is toxic to fish, but there are two ways to remove it.
Plants take up ammonia as fertiliser and if there are lots of live plants in the tank, they will remove all the ammonia made by the fish.
If there are no live plants, bacteria 'eat' this ammonia and turn it into nitrite, which is also toxic. More bacteria 'eat' nitrite and turn it into nitrate. We can grow them before putting fish in the tank by adding ammonia to feed the bacteria, but this takes weeks to do. This is called fishless cycling.
If fish are put into a tank that has no live plants and the bacteria haven't been grown first the ammonia will build up in the water because there's nothing to remove it. In this case, the water must be tested daily for ammonia and nitrite and a water change must be done every time either of them read above zero. It can take several weeks before they both remain at zero.