Water tank cleaning- 1 fatal, 3 hospitalized

https://www.thestatesman.com/cities/worker-dies-after-inhaling-toxic-gas-while-cleaning-water-tank-3-hospitalised-1503587079.html

Water tanks are presumed safe to enter, but they are not.

One contract worker entered the Water tank for cleaning,  died of asphyxiation. 3 more entered to rescue and lost consciousness. 

Please go through news report in the above link.

Oxygen levels will come down due to microbial action in water tanks. Oxygen levels shall be checked at actual place of work before entry. Better to use self contained breathing apparatus along with portable oxygen detectors and follow buddy system.