The recent heatwave has led to tens of thousands of fish dying in rivers and lakes, 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.
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 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%.
Geoff Bateman, head of fisheries and biodiversity at the Environment Agency, said: "Long periods of hot weather with low rainfall can be deadly for fish.
"The Environment Agency has a 24-hour incident hotline on 0800 807060 and we encourage anglers and people out enjoying rivers, canals and lakes to call if they see fish gasping for air.