New tank syndrom!
It happens all the time to new aquariest.
First off, the tank needs to go through the first nytrogen cycle. This is when all the extreme levels of amonia, nitrites, and nitrates rise and fall to level out hte bacteria growth in a tank. With all the fish you introduced to the tank, it exploded with a toxic level of amonia and nitrates that sufficated the fish.
There are several ways to cycle a new tank, and I would reccommend that you hook up with an experienced aquatics person in your area to actually help get you going.
In my past experiences, we use to take water and gravel from other tanks to start a new tank. This way the water and gravel are what is called, or refered to "as being seeded with the nutrifieing bacteria". Nutrifieing bacteria is the balance of organic materials and biological life forms that help the decomposing elements of a tank. Fish droppings and excess food that was not eaten.
The new practice is to put in your water, dechlorinate it with the chemicals, through some added junk into it and let the bacteria grow in it for a couple weeks. This helps cycle the tank with out creating a melt down with your investment of fish.
Or, for a more faster result, get some gravel from a already seeded tank, half the water and their filter pads. Let it grow in your tank for a couple days and then test the water.
Either way, you need to have some help from an active aquatics person. I also reccomend purchasing some books to learn the basis and how to distinguish diseases and such. I have a librery of more then 20 books. Books are great to help teach the family.
I have been doing this for more then 30 years and still call myself a novice
Good luck
Maximo
Help My Fish Are Dead 
