1. You may have six mullets at the middle left.
2. You may have six mullets on the middle left.
3. You may have six mullets in the middle left.
4. You may have six mullets at the left middle.
5. You may have six mullets on the left middle.
6. You may have six mullets in the left middle.
7. Can you see a sea anemone at the center of the picture?
8. Can you see a sea anemone at the middle of the picture?
9. Can you see a sea anemone in the center of the picture?
10. Can you see a sea anemone in the middle of the picture?
(Hello. Good to see you anain. One more question about prepositions: Which prepositions should I use? Are they all acceptable?)
In sentences 1 - 6, I'd say #2 is best, #1 is next best, and the rest are about the same. Sentences 4 - 6 are a bit awkward with 'left' before 'middle.' I can't explain why; it's just the way they sound when you read them aloud.
In questions 7 - 10, only #8 sounds off. The others are fine.