everyone has the odd un explained death from time to time so don't be too hard on yourself. It certainly sounds like you've been doing things right, sometimes the fish is just the runt of the litter so to speak and is susceptible to diseases.
Finish the course of meds just to make sure you eradicate it from the tank.
HELP!!
The tank water is now tinted red in color. Another one of the cories was laying on top of a plant near the top of the tank and swam a little when touched with the net. The water levels are:
amonia 0
ph 7.6
nitrite 0
nitrate 0
This morning it was still a little yellow which was assumed due to the meds since that is what it had been earlier this week.
Don't understand at all!!