I recently treated my FBT tank for worms. I do not know whether or not they were parasites. But I spotted three sorts of worms. One species was very small, maybe as you describe. One species was much longer and like a filament. And one was a planaria.
I added a dose of Fenbenzadole that I purchased from Amazon to the tank. All worms dead in five days. Did no harm to the shrimp, snails or toads nor other microscopic organisms, but it eradicated the worms in a very targeted manner.
Search for "fish fenbenzadole" on amazon. Kills worms dead.