Tiger barbs in community tank help!!

My fiance had tiger barbs in his tank and we had a couple of guppies in there as well. We woke up the next morning and the guppies were dead so these little guys can be quite aggressive.
 
I just started a tank over the summer...I knew from the beginning I wanted Tiger Barbs in the tank...Told the guy at the pet store about it...So I started off with 3 black skirt tetras to start out the tank...Then got 2 more and 3 Australian rainbow fish...Finally added 5 tiger barbs.....They dont really nip at all...I think its because there are 5 in there and the other fish were already in the tank a while before I got the barbs
 
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