Thanks for your replys. They were together for just a few weeks. This Niger was a little bitch (and I thought they were nice), though. As soon as I put him in the tank, he picked a fight with my ET puffer, but got his butt handed to him on that one. So, I thought that may have put him in his place, and he was quiet for a little bit.
If you look at his right side fin, you can see there are some parts that were bitten off. What would a secondary infection look like?