hey dude, calm down. seems like your fighting the assistance given. brands was capped because i dont know what meds contain what. i just know what to look for for specific diseases. im 99.999% certain that is parasite infection. whether camallanus or some type of flagellate, or even protazoa. thats why a broad spectrum would be better than what youve been usuing. Levamasole is good stuff. starve the fish before you feed it. prazi is good too. if you can find a med that contains levamasole and praziquentel (or even nitrofurazole-sold for bacteria and even fungus but can be useful for some parasites.)
do you have live plants? what sort of substrate? are you certain the worm you saw isnt a tubifex? ive got them in one of my tanks. some of them are really small and sort of see through as you described. glassworms...or camallanus. all sorts of worm-type things that can get into a tank.
if this were bacterial, there would be other symptoms by now, as you said this has been going on for months.
you will also need to treat the water for parasites. there are meds you add to the tank. doesnt harm the filterbacs but is essential to eradicate the parasites (depending on which type you have, some have a complex life cycle and free swimming larval/"juvi" stage. it may explain why youve had reoccurance of this illness. and parasites dont always make fish look skinny-some make them very bloated and red JUST like your picture.
i know it is frustrating for you. id be going nuts. have you done any side-research? im telling you-google internal parasites and read read read. look at images. you will get an "a-ha" moment eventually.and as i said before, camallanus isnt always easy to get rid of. if you lose all these fish, i suggest not getting any more of that type.
not trying to argue or anything bear in mind. i dont envy your situation.
best of luck
CHEERS