Here are the correct choices to fill in the blanks:
6. **A. with**
7. **B. take up**
8. **A. number**
9. **B. Compared**
10. **D. although**
So the revised text would read:
"It may be surprising to know that fish are a popular pet to keep besides dogs and cats and keeping fish becomes increasingly popular **with** several reasons. First, fish are quiet and do not **take up** a lot of space. Unlike dogs and cats which make a lot of noises, fish hardly make a sound, and it doesn't require much space to keep a fish. You just need a tank of a size that fits the **number** of fish you want to keep. **Compared** to other animals, fish are also cheaper to keep. For families with young children, taking care of fish can teach the young kids to be more responsible. **Although** it's not as time-consuming as taking care of dogs or cats, keeping fish can still keep children stay interested."