I was about to ask a bunch of questions but reading the condition is quite bad, the vet sounds like the best idea, provided they know about fish.
Else some things you can do, if not covered already, are
- to raise the temp a bit while keeping the tank well aerated,
- to keep doing water changes, maybe even daily, while replacing the aquarium salt
- to test the water (what are the readings for ammonia, nitrite, nitrate if you did already?)
- to feed them a good, protein enriched diet (high quality pellets, frozen food, earthworms, crickets, insects, whatever)
- if using feeder fish, to stop that for now
- to take the carbon out of the filter if using any
- to add a vitamin supplement to the food, provided hasn't gone off food already - if you can't get any for fish, some for kids is possible as well, but without sugar
Can't think of more right now. There are lots of theories on what causes HITH and probably different possibilities as well such as bad water conditions, malnutrition, internal parasites (Hexamita or others).
If you manage to get meds from a shop, don't let them you into something "that works for everything", those do more harm than good and will weaken the fish even more and neither work with hexamita in most cases, should it be that. Meds are toxines after all.
Good luck and keep us posted!