I’ll repeat my oft repeated advice
Daily water change - this really is the most effective “medicine” you can offer: most OTC remedies are of limited efficacy against internal pathogens, and especially anything bacterial
OTC external parasite remedies also have limited effect - easy to treat “ich” (except for the drug and heat resistant strains (which come out of the intensive aquaculture industry)), difficult to actually cure “velvet” (fish that actually recover have very mild infections and remain healthy enough to mount an immune response)
Goldfish seem to harbour ich (and other external parasites) over longer periods of time, so expect to repeat/continue any treatments
eSha is not available locally so I’ve no experience with their products (looking at ingredient lists, some are “hopeful” supportive remedies (usually a response to regional legislation for active ingredients rather than a company being “weak” in their science .... though there are also some Magic Elixir brands on the market))
If possible consider some goldfish specific remedies
I’m guessing from fish behaviour that there are still external parasites present - have you ruled out lice, anchor worm etc?
Better Guessing comes out of more detailed tank history, fish history etc
GF generally do better in at least moderately hard water
Salt can be an effective “assist” for GF
Epsom salt can help to reduce inflammation
Feeding inappropriate food to GF really is stressful to their digestive system and overall immune health - better to skip the Tetra Flakes and offer suitable foods (lots of info on GF lists/forums/discussion groups etc) or NO FOOD (GF manage minimal/no food for extended periods)
GF are inefficient and struggle due to their body shapes - they are sensitive to oxygen levels (while managing to endure significant ammonia burn - which usually causes permanent damage to gill tissue, thereby increasing oxygen sensitivity), so decreasing water temp may be a consideration (I have no idea what tank temp may be)
The eye is obviously a site of bacterial infection - likely both external and internal - if you’ve a local vet, Baytril injections have shown good response (but generally require more than a single injection, especially as this is an infection of some duration)
If fish is actively eating, select a suitable food, source a suitable antibiotic: combined antibiotic~food + bath treatments may effect a recovery (but is much less efficient than internal injections and usually require some repeat treatments as the infection resurfaces, treatment should begin as soon as there is the slightest “clouding”/red blush etc)
The “healthy” fish is obviously just that, a sturdy young GF
The affected fish may have internal issues which make this fish more susceptible to any challenges - water quality, food, temperature, etc
The tank looks to be very small (even young/small GF should be in 20gal water), clouded water indicates poor water quality (bacterial overgrowth consumes significant oxygen and may release potential “irritants/toxins”) and this can be difficult to clear if “green water” related
If you can’t change many things, I’d be inclined to euthanize the sick fish, thereby maintaining the continued health of the presently non-sick-fish ... in a larger tank, with optimum water quality, suitable foods and temperature it’s likely the healthy fish will be less challenged by the presence of the considerably ill fish (which will have both external and internal pathogens/parasites present at high levels ... which is why frequent water changes are so important)