What types of tubing can I use with Push-to-Connect fittings?
You need to use tubing that is rated for hydrogen gas. McMaster-Carr may have some. Likely you'll have to use stainless steel or an exotic plastic like teflon. Normal hardware-store tubing will react, possibly violently, when used with such fluids/gasses in them. If you're cracking water into hydrogen / oxygen, that line will need to also be customized for carrying mostly pure oxygen.