Please answer all of Omorrokh's questions. A 50% water change would be a really good idea right now, but please post your tank readings before doing the water change. Fish go to the top when they have trouble breathing. This usually means that ammonia or possibly nitrites are high. If you nitrates are sky high, this could be the problem also.
Your tank is not cycled yet. When you add fish to a tank, they produce ammonia as waste, which is extremely toxic. Bacteria grows in the filter which converts thaqt into nitrites, which is also toxic. More bacteria grows and converts that into nitrates. These are not toxic until they get over 20--weekly water changes keep them in check.
Because you still have ammonia, the tank is not fully cycled yet and the fish are a bit stressed. Test your levels daily and if the ammonia gets over 1, do a water change. A cycle usually takes 6-8 weeks, though it mayh take longer with fish (people are starting to do a fishless cycle now using flakes to prevent fish from being harmed), so you should be on your way.
My apologies if I am repeating stuff you already know...I just tell everyone to make sure. In short, the answer to your question is the fish are hanging at the surface because of the ammonia.
What and how many do you mean by assorted tetras?