The Texas Parks and Wildlife Department (TPWD) confirmed the results of their testing of the water in the Concho River along Mullins Crossing Road in Miles.
Golden alga blooms were found to be the cause behind the number of dead fish in the river.
When in bloom, it releases toxins that can kill fish.
The TPWD tells KIDY the blooms have stretched along the Concho River and through the Colorado River since March.
The TPWD also says there was no evidence of oil or septic spillage.