Huh. I'm not saying they aren't sick, they might be. Sorry I totally forgot about the other thread.
When you bring the fish back to the store, can you ask for store credit instead of getting another fish right away? Might not be the best idea to keep getting more if they are having problems with their own fish at the moment.
If can't afford to treat them (you can try aquarium salt, but tetra are sensitive to salt), I'd just wait this out. Don't increase temperature because that'll only help the fungus grow.