I bought a 20 gallon aquarium about 8 weeks ago.  I have tried 2 times to put fish in and they have all died both times and I am about to give up.

The first time, I filled it with water, use the conditioner and let it run for 3 weeks.  Then I added 8 neon tetras and a dwarf gourami.  They seemed fine at first, but then they died one by one over the course of a week.  So, I emptied it out and filled it back up, use more conditioner and tried again, but this time I waited 4 weeks.  I tried just 5 neons this time to not lose so much money.  Again, they all died within a few days.

Is my tap water bad?  Can I do anything to keep fish?