Came across your post, I'm quite new to this site, but what an odd problem your having.
What else is in your tank? Is all other life doing fine? Sometimes the life in our tanks can survive rather well in the environment that they "grow up in" and live in.. when you ad new life that dies off that fast, seems you need to question a false reading of some sort.
You have a relatively new set up as I do, have you added anything to treat an ammonia spike or such? If so perhaps some of your tests are not reading correctly, some additives can not accurately read through many test kits and give results that turn out to be very wrong. Perhaps take a water sample to your LFS
and see what results they come up with on a full water test.
Good luck.. I feel your pain. Please let me know what/when u find the solution to your problem.
May your tank prosper.. ~WildFlower~