I would say that you need an all-year round treatment that also includes a heart worm preventative. Heart worm is a horrible problem in Fla. My aunt's cat had heart worm after a few months after they moved to Fla from Va.
There are inexpensive places online that you can get the preventative from. Vets will charge you 3x what you would pay online.
Get a fine-toothed flea comb (the expensive kind from the pet store, marked as a flea comb, with metal teeth) and comb the cats. If they have fleas, you will find them. Comb them every night. If you find fleas, treat the cats.