That is a key symptom for what we term "Unknown Neurological Wrasse Disease".
Sorry, there isn't a cure known as yet.
Unknown Neurological Wrasse disease (UNWD)
There is a disease that afflicts newly acquired wrasses, usually Cirrhilabrus fairy wrasses or flasher wrasses, Pseudocheilinus, but occasionally other species as well. The gross visual symptoms are always the same; the fish shows a rapid onset of neurological symptoms where it either cannot swim well, swims tail down or swims with a bent spine. The other key symptom is that despite this, these wrasses will still attempt to feed. Most aquarists attribute the symptoms to some sort of an injury, such as running into the side of the tank. The trouble with that diagnosis is that these fish do not show external damage (bumped snouts, etc.) that would be expected from such an injury. Other hypothesis include barotrauma from deep water collection, or the use of cyanide to collect these fish. The trouble with these possible causes is that symptoms in other types of fish are known, and are different; emaciation in cyanide collection and swim bladder issues in cases of barotrauma. Mycobacterium has also been implicated, but that issue is more often present in long term captive fish, not newly acquired ones as in these instances.
Eventually, the affected fish becomes weaker and either needs to be euthanized, or dies spontaneously. To date, there is no known cure and the mortality rate seems to be 100%. All that can be said is that it is an unknown peripheral neurological disease. It does not seem to be highly contagious from wrasse to wrasse. The causative agent is possibly viral, but could also be nerve damage from nematodes or from microsporidians. Since the fish still tries to feed, it is unlikely the brain is affected, more likely only the spinal column or muscle nerves.
Jay
