What is wrong with my tank? water is perfect?

I have a 30 gallon tank in my room. My fish have started dying lately. I really dont know why because I have been testing my water everyday for the past 2 weeks and ammonia-0 ppm, nitrites-0ppm, nitrates-0ppm, and pH-7. My temperature is also at 78 degrees for my tropical fish. I had 7 tiger barbs, 6 zebra danios, 3 platys, and 4 cory cats. It has been up and running since easter and i did a fishless cycle before that the correct way. (experienced fish owner) and the tiger barbs and danios all died in the passed 2-3 weeks.... i dont know why and what i should do because everything in my tank is perfect AND none of my fish are/were sick...it was just like i woke up and a perfectly healthy fish had died the next day...PLEASE HELP...any suggestions???