I agree with Melosu58, even without having a fish in there I wouldn't use boiling water. Your tank will notice the change between boiling and hot tap water when you eventually do have the fish in there. I would just add warmed water and constantly change the water. Make sure not to do big massive water changes. I am not sure how big your tank is, but I would only do like a half gallon each hour or so. Hope this helps!