Up to 5,000 dead fish have been pulled out of the Birmingham Canal Main Line after the water became contaminated.
And British Waterways today warned they expected the problem to get worse before they could get it under control.
The deaths are believed to have been caused by chemicals running into the water after an arson attack at Residual Brand Management, in Ettingshall, last Monday, reducing the oxygen content in the water.
The arson-hit buildings, in Millfields Road, contained aerosols, paints, oils, thinners and detergents as well as shampoos.
The contamination has already travelled more than three miles, from Ettingshall to Wednesfield.
Environment Agency officers have been pumping oxygen into the stretch of polluted water in a bid to control the contamination. Source