If the Unit is Untenantable...
If the unit is untenantable, the tenant can actually vacate the premises and will not be liable for rent for the period in which the dwelling unit remains uninhabitable under Florida Statute 83.56(1)(a).
If the Unit is Habitable...
If the unit is habitable and the landlord makes a reasonable effort to cure the violation, the tenant must still pay, but may reduce the amount due for the period of noncompliance "by an amount in proportion to the loss of rental value caused by the noncompliance." Florida Statute 83.56(1)(b).
7 days Notice
The tenant must give 7 days notice specifying all of the landlord's noncompliances. However, the tenant must also state his intent to withhold rent, see Florida Statute 83.60(1). Also, the notice must be submitted at least 7 days before the rent is due to allow the landlord to cure the problems.