I just tackled these guys. Admittedly I caught them early. Use flatworm exit and follow the directions very closely.
1. Mix 50 new water.
2. Siphon out as many of them as you can. Do this over a few days leading up to water change night, not minutes or hours. I used a piece of rigid air line with silicone airline on the back of the rigid.
3. Have carbon ready. I don’t have a reactor so I ran a aqua clear filter with a mechanic bag of carbon.
4. on water change night, plan to monitor tank for hours. dose recommended amount and siphon out every one you see whether dead or alive they are what will kill your fish.
5. Do the 50% follow up dose as recommended in the instructions. Even if you think you are clear of flat worms. Remember flatworm exit is safe. It’s the dead worms that are toxic.
6. After 6 hours of monitoring and siphoning out flat worms, do your normal water change and reserve the remainder of you mixed water in case there are adverse reactions to the flatworm toxins.
7. run carbon.
I made an all day ordeal out of it because I didn’t want to lose stock or have to retreat down the road. Better safe than sorry, right? Regardless, this stuff worked and none of my little aquatic animals were harmed.
if you end up not needing the excess saltwater, save it for your next water change.