Up to 5,000 fish could be affected after oil was poured into a well in Banham.
Fish have already been killed in a nearby lake and police are appealing to the public for more information.
It is believed between 3,000 and 5,000 could be affected.
The incident is thought to have happened between 6pm on Friday, November 22 and 10am on Monday, November 25, at a well in Heath Road.
The victim discovered oil floating on the surface of his fishing lake, supplied by a freshwater well nearby, and a number of carp lying dead on top.
On inspection of the well the victim noticed a strong smell of oil and saw four to five inches of oil on top of the water.
Despite a nearby oil tank there is no evidence of oil contamination on the ground between the tank and the well, indicating that the oil had not seeped from the tank.
Police believe oil was stolen from the tank, owned by the victim, before being poured down the well.
The well and lake now require cleaning out, while the victim has lost a significant amount of oil.
Anyone who has any information about this incident in asked to contact PC Andrew London at Thetford Police Station on 101 or Crimestoppers anonymously on 0800 555 111.