Zero nitrates would indicate an uncycled tank, which yours doesn't sound like it is since it's been up for two years. Those nitrate bottles need to be shaken like crazy or you won't get a reading. Why don't you test again but shake them a lot this time? If you still aren't getting a reading, it's probably a problem of an inaccurate test kit. Most of us around here use the API Liquid Master Test Kit, which tests for all the important stuff, including ammonia.
What sort of filter do you have? Regardless of how good your filtration is, that's a lot of fish to add at one time. Your tank probably had an ammonia spike which killed them.
One other thought, there may be something in your tank that is killing them, such as a chemical or a disease. How long ago did you last have fish in your tank? What was the stocking and do you know why they died?