Jay, I haven't heard that some theronts never go into the water and I find that interesting. They must breed some time but I don't know. If they never go into the water column, how will fallow work?and some theronts never go out into the water column.
I have used a diatom filter for decades and so far have never found it not to work. But the OP already used fallow with medication and I feel the fish are already weakened from all of that. As I posted I found the tank to bare and hiding places will help but of course not cure anything.
I feel a diatom filter by itself is the best method for removing or severely lessening the amounts of any life form larger than a bacteria as long as it is used correctly. If it is used in conjunction with copper, something I have done but I don't like to use it, it will eliminate any solid pathogen in a very short time.
In the 70s I experimented with using Quinacrine hydrocloride and Chloroquine phosphate as per "Edward Kingsford, MD Treatment of Exotic Marine Fish Diseases" 1975.
But I found them inadequate alone. I discovered on my own that if I mixed the recommended dose of copper "in conjunction" with the Quinicrine ( 8- 10 mg/gal) I got "cure" or at least no visible signs of disease in a little over 24 hours. But I left the fish in there for 10 days.
I used that mixture on my tank and a few aquarium stores and it always worked. Those are as you know old medications and maybe the Chloricrine is better if not the same thing. I don't know.
The quinicrine will stay with the fish for a week or so as the fish may get a slight yellow tinge which is good. Those medications are affected by light so the tank needs to be slightly darkened for a few days.
But being prescription drugs for malaria ( a similar parasite) I can't get it any more and luckily I have not needed it.
Jay, I realize these things are old school, but I am old and I feel that lately we have gotten away from the early medications that worked so well and still work with malaria. I am familiar with them because they gave me chloroquine and primaquine in Viet Nam. Now we find those drugs gave us PTSD,Tinnitus, vertigo, depression and an entire host of other things but I don't think PTSD would affect a fish that much.