I've noticed that my comet has this small ulcer type sore that's on the edge of his tail fin. He's had it for about a year, just a white spot with the occasional "flareup" when it gets a little red. Recently it looks like it's starting to get worse, and I'm starting to get worried since it's not going away, and I can't find anything about it to begin how to fix it. Other than salt/fresh water for fin rot and ich for another fish, I've never had to medicate before, and I don't want to just jump into it. The only changes I've done was a switch to sand substrate from gravel about a week ago, and we moved last July, but looking back at past pictures he had it before then. Water parameters are fine, it's a 40 gallon tank has been cycled for about a year and a half (had the tank for over 2 years, planning to upgrade to a bigger one in a couple months). 0 ammonia, 0 nitrite, ~10 nitrate. Weekly water changes alternating between 30%/50% with prime.