The deceased were identified as Sajal, Shihab, Mukul, Shahin, Pritom and Ganesh Chandra. All of them were residents of the upazila.
Kiron Kumar Roy, officer-in-charge of Akkelpur Police Station, said a worker entered the under-construction septic tank around 9:30am.
After getting no response from him, five others entered tank one after the other and fell ill, he said.
Police and firefighters later recovered their bodies and sent them to a local hospital for autopsy.