The Environment Agency and Thames Water are investigating the sewer network to see where the pesticide entered the system, in an attempt to identify those responsible.
The campaign group Action for the River Kennet says the waterway is continuing to decline, ten days after it was polluted.
Public Health England and the Food Standards Agency have said it is safe to enter the water and eat fish caught there but the campaigners say the river is deteriorating.
Dead crayfish have been found and brown algae is spreading, killing river plants and reducing food sources for fish.
– Charlotte Hitchmough, Director, Action for the River Kennet
Fish will be looking hard for alternative food. They can be quite adaptable and will be altering their diet to include bugs which fall into the river from trees, and even smaller fish, but we are expecting to see some fish deaths caused by starvation.