Black mold in a toilet tank could be seen as being the result of a tenant's failure to sufficiently clean the fixture which would mean there would be no liability on the part of the landlord for said mold. As far as built-in air conditioning units are concerned, what the lease provides regarding maintenance of the units will determine whether or not you can hold the landlord responsible for eliminating the mold. If under the lease, the landlord is responsible for changing filters and servicing the units, then you may be able to hold the landlord responsible for remediating the mold found therein.
That being said, you must notify the landlord of the problem in writing, via ordinary and certified mail, return receipt requested. You must give the landlord a reasonable amount of time to remedy the problem and if the landlord fails to take any action, you then have the option of undertaking the work yourself and deducting the cost from your rent, or withholding rent until the problem is fixed. Remember to keep all receipts if you have to hire someone to remediate the problem or to notify the landlord why you are withholding rent.
I would need more information before I could provide you with a proper response. However, personal injury mold claims are very difficult to prove. There are only a few types of mold that can actually cause serious health conditions. That being said, if the mold problem is a result of a faulty condition in the unit that was not caused by you, you may have some recourse. I suggest that you speak with an attorney to discuss your options.
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