Just for a reference in the future. . .By posting your question and concerns and having replies and replies to those replies, it looks like, from the outside that you have gotten help. Until I happened to look at this thread did I realize that you hadn't gotten any help. What I'm saying is that, traditionally, it is best NOT to have a long thread where you're not getting help. Me and others just look for threads to help, where there are no replies.
Anyway. . .Copperband Butterflyfishes can vary greatly in acclimation and suitability for aquarium life. They are attractive, but the kind of fish I'd recommend for a seasoned aquarist only. A seasoned aquarist for instance would not have put that size fish in your aquarium; would have realized that water quality was very important to this fish; and would have taken the steps to assure the best diet and nutrition for such fishes (such as found in this post)
http://www.reefcentral.com/forums/showthread.php?s=&threadid=785228
These fish are not tolerant of swift or sudden pH changes. Before you did your water change, did you adjust the pH of the new water to the pH of the tank water?
Not very many fishes are being captured today by the use of cyanide. It's not impossible, but before you come to that conclusion, you need to investigate and learn what waters the fish came from. The only way to conclusively determine that a fish has had cyanide poisoning is through a post mortem examination. I've done dozens. But without this, the next best thing is to determine where and how it may have been caught.
I don't do a lot of alkalinity or calcium testing myself because I have a FOWLR system. But I do check each anyway because I wish to maintain my coralline and other calcareous growths.
The use of tap water is not recommended. The government/source plant that supplies your tap water is allowed to supply a wide range of water quality. Even then, they sometimes exceed contamination boundaries more often then you would think. You could invest in a good deionization and carbon treatment water purification system. Since your tank is the size it is, you might even consider just buying RO/DI water or distilled water for your tank use.
Having said all the above, from the description of your system and your observations, it seems that the fish was sensitive to something that came in with the last water change. Not all fish have the same level of sensitivity when it comes to water quality. So having one fish look bad and die while others live, is not that unusual when it comes to water quality. This is the main reason why this fish is best kept in stable water of the highest purity, with the highest volume (larger tanks) by experienced aquarists. You'll find the 'odd fish' that is mistreated and yet seems to live on and on!
I'd recommend collecting up all the hermit crabs and replacing them with snails. The crabs are omnivores and will eat anything they can get their claws on, including 'sleeping fish.' In the size tank you have, they are too close to the other inhabitants.
As for this fish, the only thing I would suggest doing is an immediate 80%+ water change using distilled water or store-bought RO/DI water to make up your saltwater. Before the change make sure salinity, pH and temperature are the exact same as your display tank water. If this helps in anyway, do this again every day for no less than 4 more days.
IF you see ANY symptoms of disease or conditions, be sure to report them. This would mean the need for a treatment of a different sort.
Good luck!
:rollface:
At this point in time I don't have much encouragement for the return to health of your fish if all you're going to do is remove the crabs.
