Dead fish have been seen rotting on the surface of Bewbush Water Gardens.
About thirty fish have been floating dead in the gardens near Mill Pond since Monday (July 6).
A passer-by said the fish, some of which were about a foot and a half long, were rotting, causing a nasty smell and attracting flies.
A Environment Agency spokesman said it is believed the reason for the deaths was the long period of hot weather.
He said: “We are aware of an incident at Bewbush pond, which we attended on the afternoon of Monday.
“It is estimated that there were 30 dead fish, though the condition of the fish indicated that they had died some time before. Oxygen levels in the pond were very low, and the cause of death is considered to be because of oxygen depletion. We believe the oxygen depletion was a consequence of an algal crash caused by high temperatures and low flows followed by thunder storms.
“The construction works being carried out by Crawley Borough Council were not considered to be a direct cause.
“Crawley Borough Council have been advised that aerators should be available, if there is a repeat of the conditions at Ifield Mill pond which is immediately downstream.
“Extended periods of hot weather with low rainfall or storms can be deadly for fish. Should local residents spot anything of further concern please report this to the Environment Agency via our 24 hour incident hotline on 0800 80 70 60.”
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