Your original sentence is: "A lionfish has thin, long fins on its back and belly that have stingy spines."
This sentence is generally clear, but we can make it a little better by changing the order of the adjectives and making the sentence more natural.
Better version: "A lionfish has long, thin fins on its back and belly, and these fins have stingy spines."
Explanation:
- We usually put long before thin in English.
- Adding a comma before "and" helps separate the ideas.
- Saying "these fins" clearly shows that the stingy spines are on the fins.