Levoit Oasismist 600S Smart Warm & Cool Mist Humidifier, 60H Runtime for Large Room

Should I empty the water in the water tank before refill it?

Using another type of water such as hard, soft, purified, filtered, or tap water in a humidifier can have some drawbacks (leave behind white dust, unpleasant odors etc.);

It's recommended to use distilled water in our ultrasonic humidifiers. Distilled water has had minerals and impurities removed through distillation, resulting in purer water. Many of our humidifiers also come with built-in filters designed to help reduce impurities. And you can use a different type of water at your own discretion, but distilled water is preferable if possible.