In most cases, nutrients from past pollution linger to flow in the pond. And you should aware that ALUM is most operative in that case, although contamination of a pond is not occurring by sediment or nutrients. ALUM is also known as Aluminum sulfate in certain industries, but the name ALUM is more commonly and properly used.

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Effective Uses of Buffered Aluminum Sulfate for Ponds or Drinking Water – Phosclear
When it comes to deal with the algal populations or to ensure non-algal controlled water clarity, Buffered Aluminum sulfate is the best option. It...