Tenant with mold issue

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I recently increased my Tenant's rent by $75.00.
About a week after the lease renewal they started complaining about mold and health concerns. Their is moisture all over the walls in some photos they sent. I have lived here myself, as well as another tenant. This has never been an issue. I purchased dehumidifiers anyways to remedy the situation and offered them to break the lease for free.

They are still unhappy and threatening me. What can I do to make this tenant happy and/or protect myself legally?

Here is what my PM Just sent me:
I had our person go through the property and clean any visible mold.  He dropped of one out of the three dehumidifiers as Walmart only delivered the one and I had to reorder the others.   I fix the mold under the sink we will need to break through the wall from the living room to truly investigate the extent of mold between the walls.   Not sure if you want to continue down that road.   Also she is still asking for the air testing to be done but I’m not to sure about doing this.   I’m afraid if it comes back positive you might open yourself up to legal issues if she turns out to have health issues. 

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