What's going on with this Tetra?

[Tacit Blues]

One of my Head and Tail Light Tetras had a very red internal organ (not familiar enough with fish anatomy to be able to tell you much more than that, but on his tank mates it is silver) when I got home from work today (the fish on the left):

The fish seems to be otherwise healthy (for now, anyway...) and doing fish-y things along with the other ones. The water tests 0 Ammonia, 0 Nitrite, and something just above 0 for Nitrate.

Any ideas what's going on?

 

[Tacit Blues]

Well, this morning he's still alive and kicking. The redness has maybe gone down a little, and it looks kinda like internal bleeding around the organs. It seems that this one and another one are fighting with each other quite a bit, not sure why.