Torfaen Council officers and the local community scrambled to save the lives of fish at Cwmbran Boating Lake after more than 100 were discovered dead yesterday morning.
Using nets, officers and residents transferred some of the surviving fish to a nearby pond and removed dead ones.
Water was then sprayed onto the lake in a bid to oxygenate the water.
The Environment Agency was called out and is now investigating the cause.
The lake has been cordoned off while tests are carried out and will remain closed until oxygen levels have returned to normal.
Christina Harrhy, the Council’s Head of Technical Services and Performance, said: “A Council officer discovered the dead fish first thing this morning and then there was a desperate scramble to save as many as possible.
“It was distressing to see the fish struggling but we are hoping the problem can be resolved quickly.
“I would like to thank the band of officers and local residents for doing everything they could to help.
“We are now working closely with the Environment Agency to determine the cause so we can resolve the problem and stop any further suffering.”
Fisherman Gareth Swithin, 43, of Coed Lee, Cwmbran, was one of the volunteers.
“I was shocked to see the dead fish while out walking my dog and I just wanted to do something to help,” he said.
“I’ve been fishing for eight years – it’s distressing to see the fish struggling.”
Tests results revealing the cause are expected in a few days.
Fish including Bream, Carp and Roach can be found in the lake.
