Raising Rent on newly acquired duplex units

[Jon S.]

I just purchased an occupied duplex, 2b/1b. Both tenants have 1 yr leases for $450/mo that expire at end of this month. As of Feb 1st, the lease will automatically convert to month to month. They are paying way below market rent. The numbers don't work, even before renovation, without raising rent. The market rent in this area, for units in this condition, is appoximately $550-$650/mo. After renovation, the market rent will be $700-$850/mo, depending on how nice the renovation is. I don't think they will be able to afford the units after a renovation and a full rent increase, but I am hoping that they can afford a small increase now. I am thinking of raising the rent $100/mo, from $450 to $550/mo. That would still be exceptionally low rent in this market. To complicate this decision, one tenant has been there 10 years, and has done improvements to the unit. Thoughts? 

I'm in Tampa, Florida, when do I have to notify them of the rent increase? Is the rule 15 days in advance? Or 30 days in advance? 

What is the best way to let them know? 

Other suggestions?

Thank you