It sounds like an allergy problem to me, I'd try going to another vet for a second opinion if they still say its not allergies and the meds haven't helped
Is there any vet at all you can get him too?
My family is on pensions too, and our vet lets us make a payment schedule in emergencies.
If their is any vet you can get him to, or even one that will come out and arrange a payment plan with that would be best for him.
Otherwise make sure he...
No, they wont eat the fungus.
And you should put the infected on in a separate tank. The others could get infected and if left untreated it can wipe out all your fish.
If they are all already showing signs of the sickness go to the pet shop and get some medicine for them, they normally sell...
I say fish are pets, but since they don't make mess like other animals can I say that they would be fine. Alot of people consider fish as decorations, I dont think thats right..but if you are unsure I would check with the person you are renting from. Just incase :)