All my fish died?

I am not at all new to taking care of fish. I had a 45 gallon fresh water fish tank for about 6 years. All but 3 fish had died from old age. I decided to give the pleco away, take the 2 little fish (guppy and silver angelfish type thing) and put them into a smaller tank. I got a Minibow 2.5. I did everything I was supposed to do. Second day, the angelfish was dead, stuck to the bottom of the filter. Another 2 days later, the guppy was dead, stuck to the bottom of the filter. So now, just today I woke up and the 6 other guppies and the little baby one were all dead. 3 were stuck to the bottom of the filter and the other ones were just laying on the bottom on the other side of the tank. I wasted alot of money on this, and did everything I was supposed to do. The place I bought all this has a no refund policy, but do you think I am entitled to a refund? What the heck is up with this fish killing filter. It's a Whisper filter by Tetra. And no, the tank was not overstocked. Trust me.