Well first you need to learn how to cycle your tank. And you need to learn to stop listening to the pet stores. They literally know almost nothing about fish, and it really is a shame. They probably told you to just run the tank for 3-5 days and add fish right? Well it doesn't really work that way. This is how to cycle a tank:
Here, read these links:
http://www.aquariumadvice.com/forums/f12/fish-in-cycling-step-over-into-the-dark-side-176446.html
Tips and Tricks For Your Fastest Fishless Cycle! - Aquarium Advice
The first one is about fish in cycling, the second is about fishless cycling. To cycle a tank you need to get some level of ammonia, and have bacteria in the tank (that get there through the air) to "eat" the ammonia, and make nitrites, which is "eaten" by another type of bacteria that produces nitrates. Ammonia and nitrites are very bad for fish, and levels over .25 ppm of those are bad and can cause permanent damage (given enough time in that water). Nitrates are much less toxic. Nitrates should be under 40 ppm, but under 20 ppm is best. Nitrates are removed through water changes.
A fish's waster produces ammonia, which is the source for fish in cycling. For fish in, you just add fish, test daily, and do a 25 % water change when water gets above .25 ppm for ammonia or nitrites, until you consistently get 0 ammonia, 0 nitrites, and x nitrates. Then, you are cycled (you have built up enough bacteria to "consume" the amount of waste produced by your current stock of fish. This is known as a bioload, the amount of waste a fish produces), and can add more fish, a few at a time.
Fishless cycling uses a source of ammonia (pure ammonia [bottled], a raw shrimp [the kind we eat], or fish food), try to get ammonia up to 4 ppm (but less is okay), and then just add more ammonia when it drops under 4, and do a water change when ammonia or nitrites somehow get over 5 ppm. Once you consistently get 0 ammonia, 0 nitrites, and x nitrates, you are cycled, and can add your whole stock at once. If you don't add your whole stock at once, say add only 5 fish, and then wait a week, you cannot add your whole stock then, as the bacteria will die off until there is just enough ammonia produced for all of the bacteria to eat. While fishless cycling you should test at least every other day.
Right now you are fish in cycling, and some of your fish may die in the process. You need to be prepared to do lots of water changes.
Also, what kind of tetras are they? Can you distinguish their species or post pictures?
Hope this helps! Welcome to AA!
