I have wiped out a tank using FWE improperly. You must siphon out as many as possible. If you have a lot, you can do this in water changes. You can also use a mesh bag tied to the siphon tube going into your sump, so you have the ability to continuously siphon. Once you are confident you have as many as you can realistically get, then do the FWE. Have a good sized water change ready and lots of carbon. Also a large fine net, as you can also net the floating dead ones (I have caught 100's doing it) so they don't poison or rot. Then, about 2-3 weeks later, do another dose of it at double the amount instructed.
The stuff works beautifully and safely...so long as you follow the directions of getting as many out manually as you can. I have done it twice since I wiped out my first tank, and it went without a problem. The first time I was careless with getting out beforehand. I have not seen a FW since
You cannot monitor the toxicity, but if you only see sporadic ones(several dozen) floating around after you dose, you did a good job of sucking them out. If you see a blizzard of them(hundreds and hundreds), you have problems and need some major water changes