You need a water test kit.
Is your tank cycled? Mass death is usually caused by ammonia poisoning.
Look up "new tank syndrome"
P.S. Do not ask people at petstores. They are out to make money. Buy a book
I personally am a fan of bare bottom tanks or medium size riverstone, but thats not for everyone :-)
Usually, I go with .5 lbs per gallon. I try to use as little bottom decor as possible because it is difficult to keep clean, and people don't realize that it limits how heavily you can stock...