Oh.
Never believe anything a shop says until you have researched it for yourself. First lesson of fish keeping.
Adding the pure aquarium may help, it may not. When they tested the water it would have shown as perfect because you hadn't yet put anything in the tank to make it not perfect - ie no fish at that time.
Unfortunately, what you were told was all too common, and you are now doing a fish in cycle. If you don't have a test kit you need one asap. You need liquid reagent testers for ammonia and nitrite at the minimum. I'm afraid you'll need to get them from a real shop as you can't wait for the post at this time of year. Most of us in here use the API master kit which also contains pH and nitrate testers.
Until you get hold of the testers, change half the water in the tank daily. The new water needs to be the same temperature as the tank, and don't forget to add dechlorinator. Once you have the testers, you need to test every day and do a water change if there is any reading above zero for ammonia, nitrite or both.
Ammonia will show up first. This is excreted by the fish, and eventually you will have enough ammonia eating bacteria to eat it all. They turn ammonia into nitrite. At first there will be very few ammonia eaters so ammonia will build up. Once the bacteria start to grow they will make nitrite so that will start to show up. Then you need to grow nitrite eating bacteria, which turn nitrite into nitrate.
You will need to do water changes as often as necessary to stop the ammonia and nitrite poisoning your fish. Eventually, both ammonia and nitrite will stay at zero by themselves, and when they've been zero for a week, then you can get more fish.
I know this link is for fishless cycling, but the first part explains what happens during cycling.
http://www.fishforums.net/threads/cycling-your-new-fresh-water-tank-read-this-first.421488/
However, if a tank has lots of fast growing plants, particularly the floating plants I mentioned earlier, they can use all the ammonia made by the fish faster than the bacteria use it.
So floating plants are the second thing you need to buy asap.
The shop will probably try to tell you this is all nonsense. Most of them don't know or don't care. After all if your fish die, you'll buy more. And so many of them do not understand about the nitrogen cycle.
As soon as possible, buy some floating plants - a lot, you can always throw some away if they grow too much but your fish need a lot of them right now - and testers for ammonia and nitrite (or a master set containing pH and nitrate as well). And do daily 50% water changes until you have them.