Just need to look up the FL laws on how much notice you need to give them that the rents are being raised and if there's any kind of restriction on how much you can raise it. It doesn't need to be in the lease, now that you're month to month. In CA, anyway, if you raise the rent, all of the other terms of the lease remain in force and the only thing that changes is the monthly amount of the rent.
Just check Craigslist, or wherever tenants look at rents where you are, and I suggest keeping your rents just enough below the going rate that they will decide to stay. Because the first thing they'll do is check to see if they can get a better deal. And, when they complain, and they probably will, you can always say that the rent is still below market. Then, expect them to come up with some stuff they want fixed. If it's reasonable, then go ahead and fix it. if they want new hardwood flooring, tell them you're sorry they aren't happy here anymore. In my experience, they'll stay, after they get over their mad, as long as it's a better deal to stay than pay to move somewhere else.
As to how to write the letter, just something like:
Dear Tenant,
As of ________, your monthly rent amount will be increased from ________ to _________, due to increased taxes and costs.
I've increased the rent to a level that is still under the going market rents, in the hopes that you will stay in the property, as you have been a great tenant.
Cheers,
You