Thanks, King Fisher. Unfortunately, it seems that things may have taken a turn for the worse.
The tiger barb in question showed signs of improvement yesterday (more activity and increase in appetite); but today all of the tiger barbs have been acting strangely. They showed some interest in food but their activity level seems to be down. A lot of them are also gravitating towards the back corner of the tank and just seem to be "off". Finally, the coloration seems a bit pale on a few of them.
Here are my readings:
Nitrate: About 10
pH: About 6.2 (It's typically this low as I run CO2
I should also note that I did a routine water change yesterday.
Any ideas what might be going on?