Treating without a positive ID is not the best thing to do, However,I would not wait for an epidemic before I took some kind of action. Catching ich early makes it far easier to deal with and reduces the risk to your fish. If that one spot is ich it will be hundreds of parasites next week. In a closed environment, that can be a really bad situation.
Are your fish flashing, or scraping at all ?
have you looked up pictures of ich to try to compare?
Can you post a picture here?
Ich looks like a grain of salt when it gets big enough to see well. it will fall off of your fish pretty soon (1-3 days depending on temp) after it gets that large. I'd try to get a positive Id right away and start treatment.
dave