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Fish were found rolling on their sides in the River Darent after a floodgate was tampered with.
Environmental officers were called to reports the distressed creatures had been found at a section close to the Riverside industrial estate in Dartford.
An operations field team later returned the sluice, a gate used to control water flow, to its correct position.
It had been tampered with and put in an up position meaning water from the Darent had flowed downstream into the tidal section.
Speaking afterwards, an Environment Agency spokesman said: "We assessed the upstream and downstream river levels and we did not observe any fish in distress.
"We received reports of fish (mainly bream and carp) in distress on the tidal section as a consequence of the sluice gate being in the upright position."
He added staff were sent to the site to assess the situation.
Darent River Preservation Society chairman Stuart Merrylees warned the waterway was already suffering after a very dry summer.
He said: "It’s sad to hear people don’t think about the effect of pulling the plug out of a river.
"It’s been a very dry summer and the flow is really low at the moment.
"People just don’t realise the chalk aquifers under the river provide drinking water for tens of thousands and they are getting very low.
"We are getting worried the world at large is just unaware how critical the situation will become."