This is a hobby still in its infancy and even maximum sizes o species is still being argued about... Some of the sudden 'mystery' deaths could have been caused by viruses or parasites but at this stage we just dont know.
Also, that is another good reason to buy captive bred as the chances of parasites etc is much slimmer... doesnt help us in europe much though!
When we get Cephs that look a bit under the weather our vet usually prescribes an antibiotic called Aquatet (Oxytetracycline-dichloride based) which is also available for acne!!! Maybe speak to your vet about this, certainly seems to turn our octos round if they have a bad shipment to us!
don't know the antibiotic aquatet, but I know our pharmacy sells an ointment called teramycine. this also contains oxytetracycline and is used to cure skinproblems. I will check with my vet if he knows this aquatet and if it's the same as teramycine.