It could be a couple things. But first, how long did you have the larger tank set up for? did you cycle it? Neons don't do well with new tanks, they prefer established tanks. That means they are sensitive to any water quality change. Perhaps your neon isn't doing well with the change in water quality (aka your newly put together tank).

Or it could be Neon Tetra disease, which has no cure. However, it is more than likely the first of the two because you just moved them and had no problems in your smaller tanks. Perhaps you can add some stress coat. Just leave him in the tank until we figure out which of the two it is. More stress on him is not necessary yet.
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Phishie

"An ounce of prevention is worth a pound of cure."

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