Fertilizer from a co-operative was linked to a fish kill along Roberts Creek a mile northeast of Elkader, the Iowa Department of Natural Resources reported.
Businesses have to report spills within six hours, but a private citizen reported the Elkader spill. State investigators found dead minnows, carp and suckers among the dead fish along a three-mile stretch of the creek.
The stream feeds into the Turkey River, which the state stocks with trout.
DNR field staff on Tuesday found high ammonia levels in a drainage ditch near the co-op, Three Rivers FS. Company staffers said they emptied a secondary containment on Monday. The company found a possible source of the spill.
The state will consider enforcement action.