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 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."
In July there were almost 50,000 fish deaths as a result of hot 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 three percent. With aeration pumps they were able to boost levels to a healthy 40 percent.
Tens of thousands of fish have died in rivers and lakes as a result of the recent hot weather, the Environment Agency has said.
Increased temperatures and low rainfall can lead to low oxygen levels in water, leaving fish at risk of suffocation or distress.
Heavy rainfall, such as the downpours which followed the heatwave, can also cause an increase in diffuse pollution and sediment washed off roads, from sewerage systems and from agricultural land, which also lowers oxygen levels, the Environment Agency said.