I don't have any photos available right now. I could post in a day or two, if it can wait.
The tort is about 6" or so (carapace length). The spot may have been there before, but I don't remember seeing it. It is directly down the middle of the plastron, but it doesn't extend the length of the shell--only about an inch long, as I mentioned. Maybe it is the normal spot? I'm not really sure...
He is active and has a typical redfoot appetite--ravenous. I am treating him with eyedrops for a bacterial problem, and he's on the last week of antibiotics for the respiratory problem he had when we purchased him. However, none of these problems appear to have affected his activity level, and he is gaining small amounts of weight each month. He was moving around a little slower yesterday, but I think it's because he ate so much the day before!
I could take him to a vet, or wait until I post pictures...should I be concerned about shell rot, or another problem? Thanks for your advice...