The fish were found in a 1,000ft stretch of the Trent and Mersey canal in Stone.
The Environment Agency is now looking into whether pollution is the cause.
The canal is near the former coal-fired Meaford Power Station where a £300 million power station is planned.
Developers acquired the site in 2004 but it has stood largely empty since then.
An investigation has been launched by the Environment Agency to find out what has killed the fish, including pike, bream and carp.
The agency and Canal River Trust spent hours yesterday (Thur) pumping oxygen into the water in a bid to save the remaining fish.
Environment Agency spokesman Hannah Brookes said:"We are currently dealing with a fish kill incident on the Trent and Mersey Canal near the former Meaford Power Station in Stone, Staffordshire.
"Our investigation into the cause of this incident is ongoing. We are also assessing the environmental impacts – our current estimates indicate that over 700 fish have died and approximately 300 metres of the canal has been affected."