Without a post-mortem on the fish (serious – see
@Paul B's past comments or ask him about it) all you can do is guess at what you could do better next time. I've seen some folks suggest blaming the fish or a mystery disease that there was no sign of and proceeding "as usual". That is not very helpful or progressive.
One factor is that 12G makes for a very small QT. The clowns alone or the tang* alone would have been plenty for the size IMO – maybe still too much in terms of stress.
Not that 3 fish is a huge number by any means, but there are good reasons to add livestock 1-2 at a time that go beyond QT size and relate to disease and system stability in the display too.
Is your QT outfitted prison style (bare w/PVC decorations) since you're using it as a medication tank too? Prison-style raises the stress level vs a naturalistic setup. Adding meds raises the stress level vs a clean, seasoned tank.
All stress factors that I would personally change next time...none is even difficult (as long as you catch the 40B at Petco's $1/gal sale):
- if you're specializing in nano-fish, that would make the 10 Gallon QT about right
- you should consider a 40 Breeder the perfect sized QT
- a 20 Gallon sould be considered small
- Outit the QT naturalistically. At least with some fake plants. Ideally with a chunk or two of live rock as well.
- Definitely has to be a well-seasoned tank....set up as if the fish were to live there permanently (if not in high style).
- Medicate only as-needed, but you can use many meds on this setup and not worry too much about the rock.
- Have a separate hospital tank like your 12G for treating with harsher medications that would nuke your nice QT. This hospital tank is the one that can be set up prison style as the fish usually won't be in it for very long at all.
I hope this helps! Please don't hesitate to ask if you have questions...you're welcome to PM if you'd prefer.
2¢
* You may already know it, but if you're successful with them and they grow up, Sailfins get too huge for a 100 gallon tank. A Vlamengi would be the only common tang you'd find that grows larger. Personally I'd recommend any of the Bristletooth tangs first as they seem by far to be the best aquarium inhabitants of the family. Depending on plans for the tank, maybe consider something like a Klein's Butterflyfish instead of the Sailfin if you still want something with "similar" colors. Klein's a
great schooler with excellent personality, but if they get hungry they do have a willingness to eat polyps (e.g. zoa's). Use an auto-feeder and keep it tuned up.
