nasty water

Question
Hi!   I have had fish aquariums before.  But have recently moved to a different state and just now set up my tank.  I've NEVER had problems like this before!!  I have a 55-gallon tank.  I set it up on a Saturday and bought fish for it on Tuesday.  I brought a small variety of tropical fish....ones that I've had before:angelfish, tigerbarbs, danios, tetras, etc.  I don't remember at what point the water began to become kinda cloudy...but I always "start" the tank with "startright", i have an ammonia test kit and treatments, and another test kit that tests for ph levels, nitrate, water hardness, etc.  When I was at the fish store, she recommended that I buy some "bacteria inducing" chemicals....similar to what I normally put in my tanks.  We added a few more fish the following weekend (1 week after I set my tank up) I had three dead fish over the weekend.  It's now Wednesday....10 days after I set it up....my tank looks and SMELLS horrible!!!!!  I went back to the fish store on Monday to ask what I should do....she didn't test my water but  "assumed" it was probably high in ammonia and suggested that I do a partial water change.  She didn't recommend that I add any ammonia-killers.  She said it was just "new-tank syndrome"....however, I have NEVER had this problem before!  So, I went home and emptied about 1/3 of the water and replaced it with fresh water.  The smell and look was better for about a day.  when I got up on Tuesday morning, my tank was beginning to look worse again (although, it never completely cleared).  So, I went to the store and bought the ammonia testers and an ammonia killer.  I tested the water....the ammonia level fell under the "stress" category.  I emptied about 1/4-1/3 of the water and put the ammonia decreaser in.  I put an extra air hose in the tank...the bottle of ammonia decreaser recommended alot of tank aeration for at least 1-hour after use.  I did all of this around noon yesterday....when I got home from school last night at 8:00, my water was so murky and nasty that you can't see the other end of the tank if you look from the side.  AND it smells AWFUL again!  I don't know what else to do.  
My pH and nitrate levels seem to be okay.  I have decreased the amount of food I feed the fish....
The fish usually seem to stay at the top of the water, almsot as if they were trying to get fresh air or something; they seem to be breathing a lot faster as well.  They don't really seem to swim erratically or anything, they sometimes swim faster though.  
Right now, almost all of them, are in a little "huddle" at the top of the tank....they aren't really swimming around, just kind of sitting at the top, not very active!
Please help!!!  If you have any other suggestions...I haven't had any fish die in 3 days....adn I am trying to save them!!  This is incredibly frustrating becasue I've never had anything lik ethis ever happen!!

Answer
Surprisingly, the fish store lady is correct.  You are going through "new tank syndrome" which is called cycling.  The first 6-8 weeks of having an aquarium is the most difficult.  You have no beneficial bacteria in your filter and as the fish poop and the food breaks down, you get a buildup of ammonia.  Ammonia is lethal to fish, and in an established aquarium, bacteria break it down into nitrite, which is broken down by another bacteria into nitrate which you remove with water changes.  During this 6-8 weeks, you should only have about 8-10 hardy fish.  It sounds like you have too many fish in too short of time.  Angels, tetras and barbs are not ideal fish to cycle with.  If they are still alive, I would recommend taking them back to the fish store until the cycle is done.  What kind of test kit do you have?  i would recommend getting a liquid reagent kit like API master kit.   If you go to petsmart.com and print their online price and take it to the local store, they will give you the online price.   Test your water every morning and night.  When ammonia or nitrite climb above .25, do a 50% water change.  Do not add any chemicals to the tank as these will stress the fish and mess with the water.  The best remedy is the good old fashioned water change.  I would go and do a 50% water change now, check the water before bed, and do another if needed then.  You will most likely end up doing at least one water change daily, esp. if you choose to keep all of the fish that you have currently.  

Here is an article about cycling and starting a new aquarium:
http://www.badmanstropicalfish.com/articles/article34.html