The man was wearing military clothes and a backpack filled with rocks, Radio Television of Serbia reported Monday. Investigators also found a rifle in the water tank.
When the man was found was not reported.
Josip Kasnar, the head of the local waterworks, said that the water had a strange taste about 10 days before the body was found.
Investigators said they are looking into whether the man committed suicide or was murdered.
Meanwhile, health officials are conducting tests on the water in the tank to determine whether it is safe for residents to drink.
Interpol investigating stolen passports on missing flight
Senate Democrats to pull all-nighter on climate change