Fresh water tropical

Question
We have had a tank set up since christmas.  Since the day we started introducing fish many have died slowly one by one.  All tests results come up at zero ppm and ph is neutral.  We have had the water tested elsewhere with same results.  The tank holds tetras, sword tails, angels and the like - it doesn't appear to be any one type of fish that is dying.  They appear to be sick and sluggish the day prior and die within 24 hours.  To our surprise the main fish dying are guppies. Any help would be much appreciated as we are at a loss. Thanks
Duane Scott

Answer
Hi Duane,
  Is there one kind of fish that isn't dying, e.g. a larger angelfish?  It is very possible that one fish is killing off the others.  

-- Ron
  rcoleman@cichlidresearch.com
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