Desperate For Aquarium Help!!?

My husband and I have always been into having aquariums in our house. We have three salt water, one with oscars, and one with exotic rare fish. We recently started a small aquarium with tetra's, mollies, and guppies for our two boys. Over the past couple weeks I have notices some tail damage on the guppies. This guy who we get our fish from said it was most likely a parasite so we started to treat our water. I have now noticed the fin damage on the mollies as well and the other day I caught a glance at one of our tetra's biting the tail of the guppies. I looked up about our tetra's(Bleeding Heart) and the fish we have don't look like the picture does anyone know what kind of fish they are that would attack our other fish because tetra's are not aggressive. I can't find any fish that is even similar to them. They are silver with navy blue top fins and a navy blue spot on both sides. and their bottom fins is outlined in navy blue. Please help my kids are very upset about the guppies.