If you want to add an antibacterial agent to your humidifier’s water, you can add a teaspoon of bleach, a cup of white vinegar, or a teaspoon of hydrogen peroxide. This will combat bacteria to prevent mold and mildew. If you want to add something to your humidifier water that will create a pleasant smell, you can add a few drops of your favorite essential oil or a few fresh slices of lemon. Remember, you should be changing your water tank daily, so don’t let anything, especially fruit, sit in the humidifier for more than 24 hours.