Filip,
I think you may not be dealing with flukes here at all. Without a positive Id by a microscope its a guess at best whats going on here. If they are flukes, they most likely didnt come from the plants and instead they were always present on the fish from when you bought them but over time increased in numbers until they are finally over whelming the fish..
Since the problem appeared to start with the plants Its likely they did carry something in... my guess would be oodinium or some other protozoan..
Pp woud.knock back a protozoan temporarily but it wont eradicate it completely.
A few things to try.. formalin plus malachite green is one option. If it can be had in your Country. Its broad spectrum against many external pathogens. Seachems Paraguard is an alternative...
http://www.seachem.com/paraguard.php
The other thing you can try which will work well on flukes also is salt dips. Remove the plants from the tank... dispose of the filter material.. place fish in a salt dip in a bucket and clean your tank out thoroughly. replece fish to a sterile tank ... repeat as many times as needed until symptoms are gone. You will unfortunately lose your biofilter but this method is very effective.
hth,
al