Possible Fish Disease

I'm heading off to the fish shop tomorrow morning, now i know i'm going to need an air stone, to help prevent the oxygen drain caused by the medication, so from the symptoms what kind of medication should I buy since I dont want to buy anything unessecery as this won't help the fish at all.
 
Would any of this be side affects of one of my neons attacking the others, since I watched it take a bite out of another neons fins.
 
It happens i'm afraid, bad water quality, plus pecking order, best to keep in a large shoal.
 

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