Solar activity was intense in September 2017 and its effects were observed in different detectors placed at the Earth’s surface. Three halo Coronal Mass Ejections (CME) hit the planet and caused magnetic storms. The effects of the CMEs on the flux of galactic cosmic rays at ground level were observed by the Tanca detector, which is one of the water-Cherenkov detectors (WCD) that make up the Latin American Giant Observatory (LAGO). In this paper we present the detection of Forbush events observed by Tanca during the month of September 2017. This WCD is installed on the campus of the University of Campinas, in Brazil, having three photomultiplier tubes that detect Cherenkov photons produced by cosmic radiation in 11400 liters of ultra pure water. We present the description and performance of the experimental apparatus and the observation on days 6th, 8th and 13th of the Forbush events originated by the CMEs. A decrease in the cosmic rays flux due to a stream inTeraction region was also observed on 14th September. These results were compared with observations made by neutron monitors and indices of the Earth’s magnetic activity.