Oh no!, Bozzzz. . . I'm so sorry to hear that you've been having trouble! I know we've been in the same boat for a while now. It's a possibility that even if your levels are correct in your water now, these fish are dying because of the stress that was put on them initially while the tank was still cycling. If I remember correctly, we both started out on the wrong foot with the same types of fish in an uncycled tank. Shopkeepers really ought to be required by law to tell people about the nitrogen cycle before selling fish!
I really hope that all the advice you're getting will help you to get this straightened out before you lose any more fish. . . *cries*
Best of luck!