if the fish are still alive, then the ammonia has rised graduallly, not in and hour or overnight. the fish will be stressed, yes, but not as much as if you change 50% a day. changing that often will also be destroying the benificial bacteria in the gravel, filter, and on the decorations, so you'll be doing 50% water changes for a very long time until the bacteria has a chance to catch up, for every time you change the water, some of the bacteria are disrupted, and changeing 50% a day will leave very little if any. changing 50% one day and waiting three or four after will allow the bacteria to grow again, and with lower ammoina levels they can keep it there. for example, if your ammoina is at 4.0 and you do 50% water change, it drops to 2.0, and then a few days later its still at 2.0, that means the bacteria have caught up