Help! why isn't my neon tetra swimming next to its conservatory?

I have a school of 6 neon tetras, but now singular 5 are swimming together.

I have read that this is a symptom of "neon tetra disease" and i dont want him to die.

is there any other reason he is doing this?
What are the Pros and Cons to...
Answers:    It could be next to out a threat it doesn't feel the need to shoal, having get a shoal of around 50 cardinals they do tend to split off a bit, of course if I introduce something for a while larger they tighten up, also I keep cardinals in the quarantine tank a. to keep hold of it loaded bio wise and because they are one of the few fish that are resilient to neon tetra disease.
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