The Parks Trust is investigating after it received a number of reports of dead fish being found around Furzton Lake.
A number of visitors to the area have reported that fish, mostly common bream, had been found dead at the southern end of the lake where it narrows into the Loughton Brook.
Parks Trust officials visited the area and took water samples which showed no sign of pollution or the toxic blooms of Blue-green algae.
Nisa Cable discovered the fish on Sunday night while on a walk with her boyfriend.
She said: “We walk round the lake regularly and we were shocked to see them out of the water. Why would someone take them out and leave them like that?”
A Parks Trust spokesman said: “We are certain that the cause of death is hypoxia, or oxygen starvation, as common bream is the species most susceptible to this danger. No bird life, invertebrates or other wildlife seemed to be affected.”
The hypoxia could have been caused by the humidity and the sudden rise in water temperature. The humidity means plants in the lake release less oxygen than normal and while larger fish can cope, most bream will die.
This is not the first time the city has seen this problem before as the same problems have been reported through the years at Willen and Wolverton Mill lakes respectively.
Most of the dead fish have been removed from the area but Parks Trust workers are continuing to monitor the situation.