What causes sudden death of newly added Zebra Danios to a fairly new tank?

Within the past month I have setup a 30gallon freshwater aquarium. I added a school of Neon Tetras (7+ fish) and a school of Glolite Tetras (about 7 fish). Yesterday, after doing some reading, I decided to get some Zebra Danios, after hearing how hardy they are. So I get about 7-8 of them, and put them in the tank (after letting them float in the plastic bag to acclimate). I get up this morning to find two dead, and one in the process of dying (it's having like seizures and such). There is no nitrite,nitrate or ammonia present, and the pH is about 7.1- 7.2. Any ideas of why these supposedly great community fish are dropping dead?