Environment watchdog Natural Resources Wales has launched an investigation after 300 fish were found dead in a stream in Abertridwr.
Officers were called to the Nant-y-Aber stream in Abertridwr on Friday September 6 following concerns from a local person about discolouration to the stream.
Upon arriving at the site, officers found approximatley 300 dead bullheads, plus small quantities of dead eels and brown trout.
The team were successfully able to trace the pollution back to the source and stop the discharge.
Natural Resources Wales will now will begin proceedings to take formal action over the incident.
Water samples have been taken to be used as evidence.
A spokesperson from Natural Resources Wales said: “This pollution has had a serious impact on water quality and wildlife in the stream, and we will be taking action against those responsible.
“At the moment, we’re continuing to monitor conditions in the stream to make sure there is no further deterioration.
“We’d like to thank the person who reported this incident to us and encourage anyone concerned about pollution in their local river to report it to our incident hotline on 0800 80 70 60.”