well once one dies, they can release some bad things that could take out a full tank. Some people suggest doing water changes when certain things die. Sometimes when you get live rock you can get some dead things inside it that can do something like this.
what all do you test for and what are the exact readings. how many of what kind of fish died. how long was it in between dead fish did you introduce new fish. thats the kinds of things people need to know to help better answer your question.
If you put any fish in there within 6weeks I'm assuming that they died because of your cycle. What do you have for a filter? 20lbs of live rock is not enough for a 120gallon tank, it would take longer for the cycle to finish with that little amount of rock. I agree tha "they're all good" means nothing, it depends on what your definition of good are... so posting them lets us know.