Investigations have shown that the pollution incident on the Afon Lwyd that saw it turn orange did not lead to any fish being killed.
But staff at Natural Resources Wales plan to survey the river again in the summer as impacts to the river may not be seen for some time.
Cwmbran Angling Club told Cwmbran Life they were worried that the mine water pollution from the Cwmsychan Brook Culvert, a former iron works site near Abersychan, could have affected fish stocks.
Holly Sisley, Environment Team Leader for Natural Resources Wales said: “Following the pollution incident that occurred on the 12 January, which saw mine water cause discoloration to approximately 20 km of the Afon Lwyd, our officers have now carried out full biological and fisheries surveys to assess for any environmental impact.
“Whilst the fishery surveys found no dead fish or fish in distress, due to the nature of this incident, it is possible that environmental impacts to the river may be seen over a longer period of time.
“We plan to re-survey the sites over the summer months and compare this data to our existing data sets at these locations to help determine if there are any impacts on any fish stocks, which will help determine our next course of action.
“Thank you to everyone who took the time to report this to us. If you see any further pollution please report it to us by calling our 24/7 incident communication line on 0300 065 3000 or using our online report it form: Natural Resources Wales / Report an incident”