Hyposalinity does not affect the theront or trophont stages of C. irritans, only the tomont. Depending at which stage of infestation the fish where aquired and placed in the QT
, it's not all that uncommon for there to still be signs of trophonts after a two week period. That would also be dependant on how long it took to reach the correct salinity for treatment.
The best advice is to double check your salinity reading and be sure you are using a refractometer. Given the innaccuracy of a hydrometer, you could potentially be at a point where it may supress the infestation some but not eliminate it. Target 14 ppt
on the refractometer so it allows for a margin of error both on your part and possible evaporation. Also ensure alk
/pH are checked daily at minimum.
Once the last "spots" are seen on the fish, start your countdown clock for 4 weeks. That should ensure you are completely successful. Once the 4 week term is up, ever so slowly raise the salinity and allow the fish to remin in the QT
at full salinity for another 2 weeks. This will give you time to monitor for secondary infection as well as determine if treatment was 100% effective.