Sorry, I didn't know your circumstances. Yes, it is hard to be put in that situation. I go by the one inch of fish per one gallon of water rule. Theoretically they would need about 23 gallons of water, you have ten, so you would need 13 more gallons.
Maybe you could get a friend with a liscense to drive you to a pet store, or advertise on the buy/sell/swap section, or just keep it. That fish will thank you, and if you keep up on water maintenance, that one fish shouldn't do much harm.
Just relax a second, and let everything settle out. see if the others get the internal parasites. If they do, unfortunately if you cannot get your hands on some medicated food, or even if you can, there is little you can do. If this is the case, you might just have to let nature take its course, then clean the tank out well and let it sit for a week or so to make sure the parasites in the tank are gone before re starting the tank. Keep your chin up, and good luck.