Nursery water is often treated with chemicals such as chlorine, chloramine, and fluoride in order to make it safe for drinking. These chemicals can be harmful to marine life, and can cause coral bleaching, damage to fish gills, and other health problems.
Additionally, fluoride can bind with calcium and magnesium ions in seawater, making them unavailable to corals and other marine organisms that need these elements for growth and survival.
For these reasons, it is best to use only saltwater that is specifically formulated for use in marine aquariums.