Now that everyone has freaked out, I would just like to point out that we don't really know the circumstances behind this. I highly doubt the fish has been in there for long, since it looks to be in pretty good condition.
I would put my money on the guy sticking it in there to keep cheap tanks.
Or he may have taken it out of a worse situation, and trying to do the best he can,
Or even his large tank may have broken, and that was all he has.
Though a 50g tank is not going to be enough for the ARTC, it would not surprise me that the guy was just bored and wanted to see what kind of squabbles he could start on CL