possible fin rot or what?

Question
I recently purchased three Black Mollies from a pet store.  They died within a week.  After that, I lost two gold fishes that I had  for over a year and a half.  Both of them were fancy types, and both ended up with ragged tails and a bleached color.  Also right after the death of these two fish, I lost my percost.  All theree of these fish were over seven inches long, and favorites.  
      The problem is that in the tank, I have eight striped Tetras, and one has recently began to have a ragged tail also.  None of the other fish appear to have any problems at this time.  I had had the tank at 80 to 82 degrees, but the pet store person told me that was too high and I reduced it to 78 degrees.  It seems my problems started after this.  Is the water too cold? or do I have other problems besides?  
    Thank you,
      Roy Landis  

Answer
That isn't fin and tail rot. That is something in your tank is higher than it should be. Do a check on ph, ammonia, and nitrites (or nitrates I can never remember which). If one of them is high, do a couple water changes over a period days. 80-82F is a good temperature for fish.