Hyacinths and lettuce are probably two of the best plants for reducing NitrAte levels. Unfortunately, both are outlawed in Florida as an invasive species. Once the additional plants get established, it should help with the algae.
Read a disturbing article the other day, claims that the reason String Algae is so hard to control is that it has the ability to pull nitrogen from the atmosphere when no other source is available.. Sounds fishy (no pun intended) to me, but who knows, could be.