Goldfish vs White Clouds

Question
I've just had a case of my goldfish killing the white clouds.  This is not simply a case of missing fish (oops gulp sort of thing) but the clouds being torn apart.

I initially had two tanks: 1 with goldfish, zebra danios and pepper catfish the other with goldfish and White clouds and pepper catfish.  About 6mths ago I merged the two tanks into one large tank.  Everything has been fine and they seemed to get along great.  There is plenty of weed for the smaller fish to hide.

Yesterday, my tank floor was littered with mauled bodies of white clouds.  Some were missing head others had be torn in half.  There are only 2 remaining alive.  What has caused this?  I've had goldfish for years and never had this sort of problem (I did have some tetras go missing, but I suspect that was simply a gulp in error!)  The pet shop seem to think it's territoriality, but 6mths down the track?

What do you think?

Thanks for your help.

Answer
Hi Prue:  Sorry to hear about your carnage. After reading your post a couple of things came to mind.  First, the scene you described and the behavior of goldfish do not match.  So for a goldfish to randomly hunt and kill a shoal of white cloud minnows would take a great deal of effort for the goldfish. Gold fish typically bob along looking for food but do not stalk and mutilate it.  They are scavengers for the most part.  It is true that some goldfish can be mean but this is more of a defensive mechanism where food is scarce.  The pepper cat would be too small to inflict so much damage on a shoal of tetra which are extremely fast.  I would consider two things.  1) tank size and 2) the amount of food you are feeding them.  A hungry goldfish would 1) pick at the other goldfish, and 2) eat the minnows not just kill them.  I have really never experienced mass killings where fish were hunted and then killed just for the sport of it.  Is it possible that they may have been sucked up the filter and mutilated? or into a powerhead?  I would keep my eye on the goldfish to make sure but this just doesnt fit the goldfish profile.  What I suspect has happened is that the white clouds died and then their bodies were ravaged by either the cat of the goldfish but I do not think that they were killed by the other fish.  Test your water and make sure the chemical levels are ok.  dave