Carp have died at a lake in Kent because the waterway dried out and could not be refilled and the fish could not be moved elsewhere.
Berengrave Nature Reserve, at Riverside Country Park in Rainham, has been hit by low rainfall and warm weather.
Councillor Ken Webber, who represents Rainham North ward on Medway Council, said the carp deaths were a shame.
The Environment Agency said moving the fish could have transferred diseases or parasites to other bodies of water.
Mr Webber formally reopened the nature reserve in April as one of his last engagements as Medway's mayor.
"We had a lot of local volunteers who got involved in making the place good again because it had been abandoned and neglected for several years.
"So it was quite a shame to find out what had happened with the lake."
Mr Webber said the water level had fallen "alarmingly", but the Environment Agency was unable to do anything and a private offer of help last week had come too late.
The Environment Agency's Miles Robinson said: "I appreciate it's caused quite a bit of distress.
"We have a duty of care when we move fish, especially to the receiving water, we have to ensure the fish to be introduced are free of disease or parasites."
Berengrave Nature Reserve - a former chalk quarry - is also now home to birds, reptiles, amphibians and rare and endangered plants following a £42,000 refurbishment.