I agree with what dave/gina said. I only suggest the early intervention with the FW exit if they do seem to be the nusiance FW's, and you start the population start to expand rapidly.
Dumping chemicals into the tank everytime you see a problem is not a good practice at all, but if you do see the plauge coming FW exit doesn't seem to be all that risky IME.
Watch the FW's closely, if you see 1 or 2 now, and a few dozen in a week or two, they're likely the nusiance ones. In that case I would suggest considering the treatment. If you don't start seeing many more of them, don't worry.
There are so many kinds of FW as greg said, many are harmless (like the opace ones Mike posted)
If you do decide to use the FW exit, or just want to have some on hand just in case, and CRA doesn't have it (?), skiptons usually does.
Just don't let the plauge get going. If you do, you'll hate it, and you'll need to spend weeks siphioning them out before treating which will be a major PITA.