Fish Suffocate Due To Heatwave

In the city in which I live we have a big artificial lake near the city center called the Aasee. It's been around since the 1930s and is a reservoir for the Münster Aa ( a little stream).The water has never been good, being full of cyanobacteria and E Coli bacteria, it is considered dangerous to your health to touch it but especially to drink it. Nonetheless the lake is home to a lot of fish. But in the summer of 2018 multiple tons of fish died. It stank when you came near the lake and all over the shore there were dead fish. Due to the heat the water got to hot, algae died and there was too little oxygen left for the fish to breath so they all suffocated.