Sounds like you did it by the pet store's book? Run the filter for X days, then start adding fish? The two weeks did nothing, in fact. Two weeks shouldn't be enough for a true cycle, and you'd need some ammonia source during those two weeks anyway (otherwise you'll get the same perfect readings you put in the tank from the tap). Also, define perfect readings - do you have your own test kit, and which one?
Here's our book, which will likely differ greatly:
Setup:
http
/www.fishforums.net/content/New-to-t...Up-An-Aquarium/
Cycling, without fish:
http/www.fishforums.net/content/New-to-t...shless-Cycling/
Alternate cycling, with fish (Not the preferred option, but it may be possible to save any living fish you still have):
http/www.fishforums.net/content/New-to-t...eady-have-fish/
Maintenance:
http/www.fishforums.net/content/New-to-t...ly-Maintenance/
Shameless plug about trusting the pet store to test your water:
http/www.fishforums.net/content/New-to-t...-Water-Testing/
(Edit: Should have broken links fixed now)
The reason one tank went bad faster could be a lot of reasons - a less hardy fish, a larger fish, or the first fish that died started to decompose causing it to get a faster ammonia buildup. Unless you left out a vital detail, chances are the second tank will follow.
??
