You won't see skin flukes on the fish. They're microscopic.
Sounds like a columnaris infection, which is bacterial and formalin won't help. Maracyn Two or tetracycline would be your best bet. Also if you do use those, monitor your ammonia and nitrite levels since most antibiotics will kill off your beneficial bacteria causing a mini cycle in the tank.
Here's a link with more info and other symptoms to compare and make sure that is what it sounds like, since really do need a bit more info to be sure, but that is a common symptom of it:
http://www.flippersandfins.net/Flexibacter.htm
Doesn't sound like anchor worms. Those will have a fork at the end of the string hanging off. And they're actually not that common. Columnaris is very common infection. Will usually show up in tanks that aren't having enough gravel vacuums performed on them. Increase your water changes and gravel vacs, for a normal stocked fully cycled tank should be doing 25% weekly partial water changes/gravel vacs. Don't overfeed either, once a day what the fish will eat in a few minutes, net out any uneaten food.
.