Hi Rachel good to see your doing the water changes.
Sorry if anything sounds blunt or rude, I'm honestly trying to help.
Their are a few problems.
The tank is quite new only being 6 weeks old.
Did you cycle the tank before adding fish?
If not then read the following article
http
/www.fishforums.net/index.php?showtopic=113861
Basicaly you need the bacteria in your filteration to remove the toxins like ammonia.
You also have over stocked your tank very quickly, this means that the bacteria in the filtration cant keep up with the ammonia produced by the fish, which is why you have the problem.
Ammonia will burn the gills of fish and will result in death if not acted apron quickly.
Personally I would remove some of the fish to allow the bacteria 7-10 days to build up to the amount of fish you have in there.
Other wise your going to need to do water changes every day until your ammonia reading is 0.
Only once it's 0 will your fish be ok. Sadly buy they the gills on most of the fish could be damaged for life.
The shop that sold you should of explained some of this and explained you should not add fish for 3-4 weeks to allow the water to cycle. and to only add 4-6 small fish at any one time to allow the bacteria to build up to the new fish.
Were here to help so check the ammonia and keep doing the water changes and let us know how its going.