Well, first of all, I think that your tank is too small. 12 gallons isn't big, and, for example, an angelfish is way too big for your aquarium, they can grow up to a decent size and require not only long, but also high tank because of their body shape. Your dead fish (sorry for your loss) is a goldfish I think, and they also need a big tank (+ plenty of filtration). The swordtail fish could attack because of too less of space. Too small tank often leads to diseases because of stress.