Just trying to Help, and Glad to be usefull. I'de Def hit the whole tank. not sure how "comunicable" Hex is but I'de personally not take the risk with parasites. And you shouldn't see any adverse reactions from your other fish to the treatment. One of the few cases where you can treat the tank and see few to no negatives to useing it on healthy fish. IDK the life cycle but with all parasite infections make sure to keep a close eye for a re-lapse in the near future. Though it's not common, I see it alot with ick.. where even a few escapes come back to re-attack the fish.
Also keep close watch on your water quality and make sure your bio-system doesn't get hit. some meds will react negatively to your BB in your set-up and jeopardize water quality. Since they can go up and down so fast and dive w/out any notice I would def test while treating the tank. Poor water quality will kill your fish faster then any disease or parasite will and will often trigger or aggrivate underlaying health conditions. So a work-up of your water perameters is essential to a solid treatment plan.
Ajno also brings up a very good point about the poop... it's suggesting more then just the Hex, white stringy poop def points to internals, Anchor worms are pretty easy to ID. I'm not sure about the meds available in the UK.. But if you can treat with an IP food once he starts to eat again I would deffinately buy some.
the LFS you bought it from claimed it was eating.. ime no LFS pays attention to every fish all the time to make sure it is eating. I wouldn't say they lied to you, but I would take the info with a grain of salt unless your experiance there tells you differently.
- Maintaining water quality
- Treating with the right doses and duration
- Getting the fish eating
should be your primary goals. a fish thats not eating ime is not likely to survive. Even if you have to resort to offering to frozen bloodworms or freeze dried krill or brine shrimp. peices of nightcrawlers also often work to stimulate an appetite. IE anything to get him eating again.
Also keep close watch on the tank for other fish showing symptoms and any aggression towards your sick fish.