Tens of thousands of fish were wiped out along several miles of a northwest Iowa waterway in the past few days. A city wastewater treatment plant is the suspected culprit, according to Ken Hessenius, environmental program supervisor at the Spencer office of the Iowa D-N-R.He says the cause appears to be the Pocahontas plant that was doing some routine maintenance that caused the ammonia level to rise in the wastewater discharge — ammonia can be toxic to fish. Hessenius says the situation is still being assessed.The fish kill stretches along about eight miles of Lizard Creek in Pocahontas County. Hessenius says the city of Pocahontas could be held financially responsible for the fish kill.He says D-N-R officials checked the wastewater at the plant on Labor Day and it appeared the ammonia levels were back where they should be.