Small fish with specialized gills processing polluted water. Presence indicates water quality survivable. Absence is a warning signal.
Small. Specialized gills filtering heavily polluted water. Visible in large numbers in shallow flooded street areas throughout the Tidal Quarter.
Presence indicates water quality survivable for direct contact. Absence indicates serious contamination — don't drink, don't wade, find another route. The most reliable living water-quality indicator in the ruins. More reliable than any device currently operating in the borough network.