I am going through a lot of water in my humidifier at night. Sometimes I need to top it up half way through the night. Is this normal?

Your humidifier setting may be too high and may need to be turned down. Mask leaks and mouth breathing (without the use of a full face mask) may lead to more water being used as more air flow will be going through the humidifier chamber (see section on mask leaks and mouth breathing).