My brother had a small school of neon tetras in with a betta fish. All of them eventually died of old age probably except for one. Now that there was only one in with the betta he began to chase him and nip at him. I moved the neon into my twenty gallon. I don't want neons, so I'm not going to get anymore. I didn't really know what else to do with him. I am really surprised because four months later he is thriving, which is really strange since there are no other neons in there. I have been noticing some weird stuff. He just keeps growing. He was normal sized for about a year in the other tank and all of a sudden he is probably 2.5 inches long! His stomach is also really big, and has been since I moved him into the twenty gal. I feed really sparingly every three days, so maybe he is just growing into the bigger tank?

could be egg laden.

could it be a cardinal tetra that somehow got passed as a neon tetra? i see it happening all the time in pet stores. it would explain why it got bigger than normal, and could explain why it is still alive and the rest arent. does the red line extend halfway up the body, or all the way from tail to the eye?