You can do 100% water change with treated water. I think most of us do 20% each time & run the garden hose directly into the tank. With minimal water changes, there's no need to treat the water although it's always a good idea.Too much water out at one time causes ich huh? Guess all my fish should getting sick because I do 90% water changes. There's also a thing called dechlorinators that remove the harsh chemicals from tap water.. If the parents are dying in the same tank as the eggs, I'd be more worried about improving the water quality than anything else.