A rare case of a fish kill due to bloom of Diatoms.
"Scientists have pegged the brown water affecting the Naples coast on a diatom, a silica-based algae, called Guinardia flaccida."
The cause of the Diatom bloom seems to be a spill of fluorosilicic acid -
"Workers at Stolthaven New Orleans LLC dumped almost half a million gallons of a chemical called fluorosilicic acid into the Mississippi River."
Nualgi results in a controlled bloom of Diatoms and hence is very useful and has no side effects.