When I had red planaria they responded very quickly. Not only would they come out but the ended up floating about the tank clearly dying. I followed the recommended dosage and instructions. I added the FWE, scooped out as many as possible, did a huge water change 24-48 hours later and ran tons of carbon. I did this for over a month and now I don't have any flatworms over a year later. This was on a 360 so I was going through almost a whole bottle per treatment. I don't think that they have built up a tolerance for FWE, from your photos I think you have a different species than the typical red planarians since they don't appear red and from your description they are brown to black. Reducing feedings might also help to reduce their numbers. The key to getting them out and keeping them out is repeated treatments and removing as many as possible before and after treatments.