Did you actually read any of our advice about cycling? Nope, thought not. Thats why they are dying, as a few of us have told you already. You need to get your water tested asap then let us know the results so we can hopefully help you sort out your mess before you kill them all. A week of having the tank running isnt sufficient to bung fish in and expect them to live!
Sorry BecciMac..., have you read something I haven't? As far as I can tell, your reply is a little out of order with loads of assumptions, how can you tell that they did not cycle the tank? If I were you I would pull your horns in and try to be constructive.
Cycling is very important, but if you have not done so, then there are ways around it. Firstly I would suggest that you complete a 30% water changes every day or so for the next few weeks or until the fish you have stop dying. Secondly I would advise that you drastically reduce the amount of food that you are feeding for the next few days / week as less food means less waste which will reduce the load.
It is possible that the reason your fish are dying is because of a lack of oxygen, especially in this hot weather. If your tank is positioned in direct sunlight then this could be a cause so I would advise that you check the temperature and reduce it by a few degrees, add an airstone if you do not already have one.
Finally, although there are plenty of potential reasons for the deaths, I would advise that you relax (slightly). Many a new fishkeeper has killed their first batch of fish with ignorance or kindness (or both). I know I did and I can certainly vouch for some of the most experienced fishkeepers who have. Your panicking may be the problem.
If you do manage to stop the deaths, wait a few to six weeks before adding any more fish and only add a pair at a time.