A. Fish often die 2 to 5 days after an herbicide treatment because of an oxygen depletion. The decomposing/rotting plants (actually the billions of bacteria and fungi consuming them) simply take up most of the available dissolved oxygen, and the fish suffocate.

Is herbicides harmful to fish?

The greatest risk to the environment when using approved aquatic herbicides is killing too much plant material at one time. This leads to a fish kill, as decomposition of plants use up all dissolved oxygen. To help avoid this, treat only a quarter of the pond at a time and do not treat during the warmest months.

Will Roundup harm fish?

Roundup is one of the more toxic glyphosate products to fish and aquatic invertebrates, such as insects and crustaceans. This is because the toxicity of a product additive allowing the product to form a thin film on the foliage of targeted weeds. ... Roundup is not approved for aquatic weed control.