Yes they can become resistant to treatment.Sorry.
These guys
https://www.google.com/search?q=saltwater+flatworms&biw=1280&bih=944&tbm=isch&imgil=1g9-FsmUDUltxM%3A%3BjpmbOzjoVg56RM%3Bhttps%253A%252F%252Fwww.youtube.com%252Fwatch%253Fv%25253DAvLSbrbIJbg&source=iu&pf=m&fir=1g9-FsmUDUltxM%3A%2CjpmbOzjoVg56RM%2C_&usg=__jj2P_JCuqk4w_D1-ZudMo-f-WL8=&ved=0ahUKEwjV7djW56rPAhWM3YMKHb9ZBLUQyjcINQ&ei=COrnV9XJNoy7jwS_s5GoCw#imgrc=1g9-FsmUDUltxM:
As I mentioned before. I was able to treat them the 1st round. Then the 2nd time I tried to treat the babies from the hatched eggs, they seems to have be immuned to the medication.
I have noticed this with the use of FWE

