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

[Luke H.]

I have a letter I send when necessary. i got it from a land lording book. It says something like, NOTICE OF CHANGE TO RENT

This is whats happening... and then it says you have the 3 options

1) go month to month at $____ (i usually put a really high rate)
2) renew for 1 year starting (date), at $_____ (I put the rate I want, slightly less than market to keep them there)
3) This is my 60 day notice that I will vacate the property on (date).
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the letter has check boxes and I send with a prepaid envelope they can send it back in. vote for my post and PM me if you want a copy of it! :)