Jay,
Of the more or less 13 common flukes, is only one visible with the unaided eye?
For monogeneans, the Capsalids, genus Neobenedenia/Benedenella are large enough to be be seen with the naked eye after a dip - they are 2 to 5mm. There are some really large flukes on elasmobranchs as well. Digeneans rarely are dislodged by FW dips (usually encysted). The rest of them, Gyrodactylids (livebearers) and Dactylgyrids (egg laying gill parasites) are soft bodied and < 2mm and are too small to be seen in FW dips. Same thing with turbellarians. What happens is that the FW turns them into tiny balls of snot, and you can't tell them from detritus without a scope (and then, not always with one). A loupe or strong hand lens might be enough to see the larger Gyrodactylids, but I've never tried that.
Somehow, the idea that you can always see monogeneans in FW dips has became firmly entrenched in the hobby. The result is, people do a FW dip, don't see anything and then think their fish are parasite-free. I'm trying to work against that incorrect information, but it is tough.
FW dips with no microscope can be used to diagnose moderate to advanced cases of Neobenedenia, but that's it.
FW dips can buy some time by lowering the parasite load.
I much prefer skin scrapes and gill clips, as the movement of the still-living monogeneans helps me see them. Obviously, I'm not not suggesting that be done routinely at home.
Jay
