Only 3 proven methods to 'cure ' marine ich .
It is not at all like in freshwater..For many it is a death sentence for the whole tank....Very contagious and very hard to get rid of …..The general cure I will guess is useless....
^^All you need to read is the second post in this link...^^
I will add on a personal note that IMO
the anthius is horrible choice of fish for almost all who mistakenly get them..They require [not like ,require] multiple feedings daily and a pretty large tank with a good flow..With anything less it usually just a matter of time before they fail and the keeper is left to wonder why....
The mandarin will probably not get the ich even if the whole tank crashes ..Some fish have better a defense then others .The fire fish and clowns will succumb quickly if it is in your tank IMO
I would treat fish with copper in a hospital tank and leave your 40g without ANY fish[fallow] for over a month at least before trying to replace any of them.
Good luck ,I have learned my lessons on marine ich the hard and expensive way....The fish that brought it via being infected or just from stressing my other fish into my 120 cost over $200 and then the whole tank[emperor angel, cheveron tang, powder blue tang] and other smaller more peaceful fish all were lost....Only survivors were a comet and mandarin, which in theory were then sub clinical carriers of the ich although reportedly 'immune' themselves for the rest of their life ?..
30 day life cycle is what is easily accepted but I side with other links stating a possible life cycle of over a year...