Oma.. Welcome to the Forums.
Your post opens a whole can of worms. First.... I'm not a doc. . I'm just a truck driver with sleep apnea who is involved with international abuse testing of cpap machines in semi trucks. So I've got experience in what you are asking about.
I will run a test cpap machine on bottled water and distilled water. I see how long it takes me to break it and why.
At home I test machines on tap water.
But to keep the legal and Irb gremlins from coming out I need to remind you that any information I give might be worth exactly what you pay for it. Nothing....
Next.... to keep the cpap machine manufacturers from getting mad I need to say you should always read and follow the manufacturers instructions for your cpap.
Ok.... now... There are other forums about sleep apnea where your question has been discussed at length.
I happen to be a big fan of America Sleep Apnea Association forums http://www.apneasupport.org/ in the Cpap machine masks. ... forum you could probably find a dozen or so threads started by other users on this topic. Stop by the Asaa forums to get a taste.
Distilled versus tap depends on how often you're going to be on changing the water, how clean is your tap water, how hard is your tap water, what make and model of cpap are you using, and do you have any medical conditions that make you catch upper respiratory tract infections easily?
The first trade off with tap versus distilled are the little bugs swimming around in any tap water. If it's pretty clean US water it's not a big deal. But if you are looking at using tap water while on vacation in Africa that might not be smart. If you dump and replace the water in the chamber every morning that helps.
I've seen some pretty bad cases of the green slymies from folks using tap water and not cleaning things.
My first winter on cpap I got a bad case of bronchitis probably from not being good about cpap cleaning.
So one vote for distilled water if you get infections easily.
The other problem is lime and hard water deposits in the humidification chamber. You can clean them out with vinegar and water. Be sure to rinse everything well.
The humidification chambers on some cpap are much easier to clean than others. ( Fisher Paykel Hc versus the new Icon) Some models have optional disposable humidification chambers (ResMed s9).
So if you don't mind dealing with lime in your humidification chamber that would be a vote for tap water.
For me I hate cleaning humidification chambers and don't like the smell of vinegar so I use distilled water whenever I can. On the other hand living on the road sometimes I run out of distilled water so I use bottled water and deal with the lime when I get home. At home I use tap water because I'm lazy and have an S9 with disposable humidification chambers.
Welcome to being a hose head.
Hope this helps.
