the exact same thing happend to me. they were right on the sand bed and gasping, yet all my levels were good. i didn't have a quarantine tank to put them in and they died within a day. ide get them to that quarantine tank as soon as possible.
there are a cupple of explinations for this. 1: if they were wild caught with cynide it could be cynide poisening. ive heard of fish being ok for a time then dieing because of the residue cynide in their body. 2: it could have been harmed by something in your tank, posibly a mantis shrimp.
i never found out what happend to mine, i hope the come out alive