my fish keep dying

I've had my fluval edge 6gallon tank now 6weeks and fully cycled I think,I added 5neon tetras 2 weeks ago and since then they have all died,at the moment I have no fish in my tank as I don't want any more deaths!my ammonia reading is 0 nitrate 10 ph 7.2 and nitrite 0.cud this be caused by a lack...