Which is not a characteristic of impressionist music?

Heavy romanticism is not a characteristic of impressionist music.
Impressionism (in music) is a style of music that focuses on tone, color, atmosphere, and fluidity. It was first introduced at the end of the 19th century by a French composer Claude Debussy. The term "impressionism" was basically borrowed from late 19th-century French painting after Monet's Impression, Sunrise.
The primary characteristics of impressionism are Subdued atmosphere, Fragility and decorative beauty, Sensuous tone color.