I don't think this hobby requires anything as much as it enables you to be a plumber, electrician, chemist, vet, etc. through researching from responsible resources. Personally, I feel empowered that by asking some questions previously I can now wire a complete lighting kit together when before I would have had to pay somebody to do it.
I think you are frustrated because you have been taken down the wrong path. The hobby is simple and should be fun, but more often than not some LFS
put sales ahead of educating the new hobbyist (if this was your resource). IMO
, they had you with a set up tank with fish before it was ready, and this and caused some problems. I would start asking questions here instead, in addition to getting a good book like Bob Fenner's The Conscientious Marine Aquarist
For this issue specifically, you are experiencing the drawbacks of plopping multiple fish into the tank straight from the pet store. EACH fish in the future should be quarantined separately for several weeks BEFORE they go in the display tank (If multiple quarantine tanks are not a viable option, only one fish should be purchased at a time.). This would have allowed you to treat the fish and possibly get it back to health without the stress of being in a new environment with other fish. At the same time it would keep the fish (and any other purchased fish quarantined) from bringing in some sort of parasite or pathogen into the display tank. Remember, even seemingly healthy fish can be carrying something!
For now, I think you are in a tough spot. Per Fishfreek's advice, it is important to know if your tank is even cycled properly yet. If not, I would suggest taking back the fish to the pet store. It is also important to know if your tank can support what fish you have/want (in regards to size of tank). If inappropriate, the Long Nose could potentially be returned as well. If not, IMO
I would move it to quarantine for observation for several weeks (this will require a Q-tank with stable water conditions!) and treatment if necessary. It will also allow any parasites in the display tank to run their life cycle without a host.
Don't mean to sound complicated, but you are now behind the 8-ball a bit. If you get out things can be much more simple and fun from here.