I agree on the possibility of older water storage tanks in apartments and flats that may be dirty or creatures died in it (remember there was local news a while back of a body found in HDB water storage tank, and the entire block of apartments were still using that water supply).
A while back i helped a friend test his tap water parameters (he lives in a 30+ year old HDB flat). His tap water TDS was unusually high, around 200+ ppm... and the ammonia was 1-2ppm and nitrates were around 40ppm, which is odd for local tap water. We suspected there is something rotting somewhere in the water supply, most likely in the water storage containers. Anyways, he got scared and end up using RO/DI unit to distill the water before usage.
Nowadays i also scared to drink water whenever i visit his house too... will always ask for can drink instead.

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