Hi catfishwoman. Unfortunately a 7 litre (1.8 US gal) tank is too small for any fish, including a betta. I expect that the person who sold you this tank did not mention anything about cycling, so your tetras would have died from being in an uncycled tank. Cycling is the term we use for growing 2 colonies of bacteria which 'eat' the ammonia made by the fish (it's their version of urine) and this process takes several weeks. If you put fish in a tank without these bacteria, the fish's waste builds up and poisons them. Unless you were doing very large water changes every day, it is this ammonia that would have killed the fish rather than lack of oxygen.
Does the tank have a heater and filter? You may be able to keep cherry shrimps in the 7 litre tank if it does have a heater and filter and you put some live plants in there. But no fish.
The first lesson you need to learn is not to believe anything a shop says until you have researched it for yourself. As you have now found out, 7 litres is too small for fish. But the shop wanted to sell you something so they didn't tell you it is too small.
I would see if the shop would take back the last tetra as it will be very stressed on its own in a tank that is too small. Cardinals need to be in a group of at least 6 in a tank at least 60 cm long.
If it was me, I would buy a 60 litre tank and cycle it
http://www.fishforums.net/threads/cycling-your-new-fresh-water-tank-read-this-first.421488/ And while it is cycling, use the time to research which fish would be suitable. Look on your water supplier's website for the hardness of your tap water - and make a note of the units as well as the number as there are about 6 different units they could use. If you post your hardness here we can help you chose fish for a bigger tank.
