The most likely reason for the pond containing full of green algae, and there are dead fish floating on the surface is "lawns are being over fertilized, causing excess fertilizer to wash into the pond, stimulating algae growth".
The term "algae" refers to a class of mainly watery, photosynthetic, and nucleus-bearing organisms that lack the real roots, stems, and leaves of plants as well as their specialized multicellular reproductive structures.
The growth of algae occurs due to -
Role of fertilisers in growth of algae in water bodies-
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