The Crystal Water Monitor app will accept any chemical and can even dose some chemicals if their active ingredient is supported by the Crystal Water Monitor app.
You can add a chemical from an action or by opening the Vessel Settings screen and scrolling down to the Chemicals section and tapping a chemical.
You can add a chemical with an active ingredient and active ingredient percentage and the Crystal Water Monitor app will provide specific dosing instructions based on your pool or hot tub's volume and needs.
If you cannot add the active ingredient because you cannot find it or because the app does not list it, you can still add your chemical by choosing 'I Don't Know' and the Crystal Water Monitor will remind you to use the chemical when needed but it will not be able to provide specific dosing.