Have you bothered to search anything before asking these questions?
3. no, normal is fine
4. Will you survive in a low oxygenated room? most likely but youll hate your life the entire time.
5. dont know, i wouldnt try it though
6. 2 clowns would need to be in a rather large tank, they will kill each other if they dont have enough room.
7. one pair pf clowns cant even survive in this
From your questions I can tell you've done no research and will most likely buy a 2 pair of clowns put them in a tank too small and come back asking why your clowns are sick/dead.
Before you buy any living animal, go to Google
and type *animal name* care, hit enter, read the first 3 articles that come up. This will give you a good idea of the BASIC requirements to keep the animal alive. If you dont like the idea of researching and learning about your pets then you're most likely in the wrong hobby.