Any of the bacteria from the established tank that was in the gravel is probably dead by now if you didn't add anything to the water. This is not a huge issue, it just means that you will have to cycle the tank with fish in it. Which means adding a few fish at a time. The fish produce ammonia in their waste (poop), ammonia will build up (watch these levels with your tester kit, do water changes if it gets too high), eventually, the nitrites will begin to build up, nitrites eat the ammonia. Nitrites are also toxic to fish, so monitor these with your tester kits and do water changes as necessary. Eventually, in an established tank, NitrAtes will start eating the nitrites. Thus you will have a complete cycle in which there is no ammonia or nitrites in the water. You will still have to do water changes to keep the nitrate levels down though. In my personal opinion, tetras are not good cycling fish as they are not very hardy. Please don't get fish "just to see how they do," you want to get something hardy that will cycle your tank for you until you can add the more fragile fish that are more sensitive to water conditions. You could start with a few danio, you can get them all different colors, they are very hard to kill and will live through a tank cycle.