Can it work last all night?
Almost, please make sure that the water in the tank is full. If you use the low mist, it can last up to 21 hours, but if you use the high mist, then it can only work 8 hours.
How do you add water?
Easy turn lock cap on the bottom.
Why I have a red light?
You have a red light for one of two reasons. 1) you could be low on water 2) you may not have the top unit on correctly, it must properly.
Dose this humidifier good for healthy?
I think it is safe for health. Until now I have used it for 2 weeks and my itchy throat has improved. And it is BPA free, so I think it is good for our lives.