Why do my fish keep dying???

Ive had a new aquarium up and running now for just over a week its been on constantly cycling and ive added water safe and liquid bacteria. I went to my local aquatic shop yesterday and told the owner all this and asked if it was ok to put fish in yet. She said it would be fine to put some quite hardy fish in to get the bacteria built up, so on her reccomendation i got 5 zebra danio's and 5 neon tetra's. But one by one they're dying and it seems such a horrible death too they sort of forget how to swim and just float around the tank bumping into things (still breathing tho) There is no obvious sign of illness, no white spot or fin rot, so what is happening to them??? And most of all how can i stop it!!!???