Sorry, tank water, the water from the tank itself. I did it during a water change. Mixed the tank water and the moss in the blender and then paint it on. I never did it in a Viv, but did it during a dry start on planted tanks a few times. It makes attaching moss to some surfaces much easier.

One of my planted tank gurus, Tom Barr, also told me about a method of using the buttermilk method on a large rock, which he then sealed in a plastic bag and then placed it inside a planted tank until the moss took over. Then we was able to retrieve it, rinse it off and placed the moss covered rock in the tank he chose.