Alum does not have a permanent effect
and it may need to be applied more
than once until plant or algal growth
decreases. A precise reduction of pH
through the addition of alum is difficult
because response is influenced
by a number of conditions in the pond,
especially the water’s total alkalinity.
Overtreatment with alum can cause a
dramatic decrease in pH, possibly to
levels more dangerous than the original
high pH problem.