Simply put, if water dissolves minerals, H20 stays clear. Minerals have electrical charges attached to their smallest components, called ions. Positively charged ions are called cations, negatively charged are anions. There can be no more positively charged ions than negatively charged ones. But, some minerals will dissolve into water and leave water in an unbalanced state.
Nature tries to adjust to the imbalance.
To get rid of the dirt, pond managers must replace it with the properly charged mineral. So the pond owner is right on track. Alum, also called aluminum sulfate, quickly alters the electrical charge of minerals in water, and can clear a muddy pond almost immediately. A mud hole can be as clear as drinking water in minutes. Alum, when it reaches the exact point chemically when mud settles, also rapidly adjusts pH. A fast change in pH creates a serious problem for fish, and can kill them.