The water you use with your MAYU Swirl depends on the type of water that you have available to you.
Ideally, you want to use purified water that has been distilled or purified by Reverse Osmosis. However, those methods strip away both beneficial and harmful components. To compensate, MAYU offers Essential and Electrolytes Mineral Drops that can be added to your water to restore the healthy nutrients your body desires (see SHOP > Essential Minerals, or Electrolyte Minerals).
We understand that only some have these types of water readily available to them.
So, you may have to use tap water.
While it is not ideal, it's still better to drink tap water that has been aerated and structured than just regular tap water.