The recent heatwave has led to fish dying in rivers and lakes in East Anglia, with officials racing to rescue many more.
The hot weather and low rainfall that gripped the UK in July can lead to low oxygen levels in water, leaving fish at risk of suffocation or distress, the Environment Agency said.
Heavy rainfall, such as the downpours which followed the heatwave, can cause an increase in diffuse pollution and sediment washed off roads, from sewerage systems and from agricultural land, which also lowers oxygen levels.
Rapid changes in water conditions leave little time for fish to acclimatise, the Environment Agency said.
There were more than 15 separate incidents in July that led to almost 50,000 fish deaths as a result of the weather, while many thousands more were rescued by Environment Agency teams and partners.
In Welney, Norfolk, routine monitoring revealed a significant drop in oxygen levels in the river, and the EA installed water-aerating equipment to improve conditions for the fish.
At Pitville Park Lake in Cheltenham, teams worked round the clock and used specialist pumping equipment to restore oxygen levels.
And in Tiptree village pond, Essex, the Environment Agency responded to reports of hundreds of fish in distress and 50 dead, and found oxygen levels down to 3%. With aeration pumps they were able to boost levels to a healthy 40%.
"Long periods of hot weather with low rainfall can be deadly for fish. "The Environment Agency has a 24-hour incident hotline on 0800 80 70 60 and we encourage anglers and people out enjoying rivers, canals and lakes to call if they see fish gasping for air. "With the help of the public we can continue to react quickly and help protect wildlife. People at home can also play their part by using water wisely; the less people take from the tap means more for our rivers and the wildlife which they support."
Pond and fisheries owners are being urged to keep a close eye on the situation in warm weather. The tell-tale signs of fish in distress are gasping at the water's surface or swimming on their sides or upside down.