i still think water quality, i just now went and checked all my tanks, all of them 0 ammonia 0 nitrite and between 5 and 20 ppm nitrate, ANY reading of ammonia or nitrite can and will kill fish, my newest 125 had a upswing in ammonia when i first started adding fish, all big hardy fish and they were stressed at just .25 for the week it went on, so, its not suprising that neons, renowned for there fragility and never ever ever reccomended for anything but the best and most well established tanks are all dying, get some cycle and some danios, wait a week, check ammonia and slowlly start to add the fish you want, any reading at all is death to fish