Why are all of my fish dying?

I am pretty sure that ammonia is the problem. Even in tiny amounts it can kill most fish. Pet stores will lie to you about that and say it is "OK" or "Not too bad". It should be absolutely ZERO or it is lethal to all but a few select species. Also, nitrites (spelled with an i) come into play. These can be lethal to many fish including the fish listed. The serpae probably has fin rot. I would try to get water quality in check with a couple water changes and then see if it clears up. I would also add Melafix as a precautionary (it is not an antibiotic so bacteria will not become immune to Melafix).