Sick GT help

When my Green Terror got sick and eventually died earlier this summer the other fish weren't affected. It was very disappointing and I think I waited too long to take action.
 
After 4 days without eating my Terror ate a spider that had fallen into the tank. Should I switch the food I feed him? I still havent found the metronidazole yet but I'm trying one last LFS before I buy it online.
 
you can try feeding him garlic.. it's a natural appetite stimulant and it prevents internal parasites.. i dont know if it gets rid of them once they're already there though.
 
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