Officers from Natural Resources Wales are investigating an incident on the on the Olway Brook, near Llansoy, where more than 1,000 fish have died.
Following a report from a member of the public, officers visited the scene and discovered the fish, which includes trout, eel, and lamprey.
Officers identified and stopped the suspected source of pollution; however the low oxygen levels in the water following the incident means fish remained in some distress.
A spokesman for Natural resources Wales said:
“We are continuing to monitor the environmental impact of this incident and are currently aerating the brook to prevent further fish deaths.
“We’ll be monitoring the river closely over the coming days to ensure that the oxygen levels in the water are high enough and that no more fish are killed.
“We would urge the public to report any fish in distress to us using our 24 hour pollution reporting line – 0800 80 70 60.”