Around 100 fish have been killed after pollution was discovered in the Afon Cefni.
Natural Resources Wales say officers were called out to a 200m stretch of the river in Llangefni on Wednesday afternoon.
The cause of the pollution has now been found and stopped while water samples have been taken from the affected area for analysis. Fish further downstream were said to be behaving normally.
But NRW added it’s still monitoring the situation following a return visit on Thursday morning. An investigation is continuing.