Would also proceed like this.
1.) Have the water quality tested
2.) At the same time, consult the tenants’ association regarding the exposed plaster.
3.) Claim a rent reduction immediately if possible.
4.) If the water sample is contaminated, in my opinion, you need advice about what you are entitled to.
5.) Take decisive action and don’t let the landlord talk you down. Why were you inactive for so long?
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On the other hand: Simply moving out in case of health problems is also a viable solution. If you don’t care too much about giving the landlord a hard time, you should seriously consider this.
Odor is caused by bacterial metabolic products. Drains can stink. Fresh water must NEVER!!!