Tests carried out by the Environment Agency have confirmed pollution was not to blame for the deaths of hundreds of fish in the Victoria Dock basin.
The fish were found floating on the surface last month.
A spokesman for the agency said: "The tests have shown no signs of pollution and we believe the deaths were caused by a lack of oxygen in the water brought on by the hot weather."
The agency estimated 1,600 fish may have died in a freshwater pond behind the old dock gates.
The deaths consisted mainly of course fish, including roach and rudd.
Bob Carter took photographs of the dead fish.
He previously told the Mail: "It is great to see wild fish in the basin, so it was very sad to see all that.
"I'm not one to exaggerate but there were hundreds floating on the surface in different batches."
Anyone who sees any dead fish or fish in distress should call the agency on 0800 807060.
