I have a school of 10 rummy nose tetra. Today, I noticed one seems skinnier than the others and is not schooling well - it sort of sticks to one side of the aquarium and will join the school if they come to that side. The tetra has normal coloration and a healthy appetite. I did not notice this prior to or during yesterday's water change. Could it be wasting disease or TB? The school was quarantined for 5 weeks and has been in the community tank for about another month with no signs of illness. What is the best course of treatment? Should I separate the one fish for treatment (would it be too stressful for it alone?) or treat the whole tank? Or should I wait and see? I have some antibiotics on hand, and Paraguard, Ich-X, and some other meds. No new fish have been added and I perform regular weekly if not more frequent water changes. Ammonia 0.25 ppm, Nitrate 10 ppm, Nitrite 0, Between soft and hard water, moderate alkalinity, and pH 7.1. Tank mates include snails, guppies, and cory cats. TYIA!