What can you add to water to make it hard enough for tea?
You are going to need Epsom salts, baking soda and Calcium chloride to make a run of the mill hard water. You can easily find these in food grade and small quantities at your local homebrewing store. I live in Seattle and we have notoriously soft water, but I don't mind it for my tea. But for brewing, I use these minerals all the time. 


Get 500ml of distilled water
Add half tablespoon of Epsom salts stir until dissolved
Add half a teaspoon of Baking soda stir until dissolved
Add half a teaspoon of calcium chloride until dissolved


Wait a bit until everything dissolves. Voila instant hard water!

If you are really serious, there are plenty of resources out there, especially around the homebrewing community. Here is an online Brewing Water Calculator