First the bad news - your tank simply isn't going to be big enough for the fish you have and you really shouldn't have more than one shark as they will (at some point) compete for territory. The fish you have are not suitable for an un-cycled tank.
The good news - you've found a place to get all the answers you need.
See if the fish shop will take the fish back.
Then get to cycling your tank - there are links in my siggy for how to do 1)fishless (probably the best option) and 2)fish-in cycling (this is what I did, but it is a lot of hard work and dedication).
Your tank won't go "stagnant" but If those fish have been in an uncycled tank for "just over a week" then there is going to be one heck of a lot of ammonia in there.
Do a 50% water change asap - remember to dechlor the new water and temp match as close as possible.
You don't mention water stats or a test kit so I can only assume that you don't have a test kit - you NEED one, the API freshwater master kit is your best option (£15-£20 on ebay, similar price in LFS).
Ammonia needs to be below 0.25ppm (at worst) and preferably 0, same with nitrItes. NitrAtes are going to be the least of your worries for now.
Read up on those links in my sig, they tell you how and what needs doing much better than I can and give loads of useful info.
