That probably explains why the fish isn’t schooling with the others. When there is something wrong with one of the fish in the school the others basically avoid or kicks the other fish out until it’s safe.
Max H is correct... if one if the tank is infected all of them are. As far as rummy nose tetras they imo are the best and tightest schooling fish. To encourage them to school better larger schools help such as 10-20. Also having one slightly bigger fish which is *slightly* more aggressive helps. For example a torpedo barb / redline shark. Still there will be times they don't always school in tight formation or break off in two looser swiming groups.
FYI my rummy nose love having their type of food changed up especially between bug bite pellets, flake food and bloodworms. They don't seem to be a fan of shrimp. This also makes their "color" very strong - bright.
Also I would return them if possible but take a raincheck and once your tank is treated get new fish. just don't bring the new replacement fish home until your tank is treated.
Well, I treated with kordons last night and then this morning my last guppy dropped a ton of fry. So I treated the tank again, added aquarium salt and prime to handle the ammonia. Going to pet smart to buy another heater and up the heat to 82. Anything else I’m missing guys?