Xiǎo yú shēnshàng de zǐyìn shì jiǎnyì hégé de biāoshì.
The purple mark on the small fish indicates that it has passed quarantine.
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usage
Not a fixed dictionary word. 紫印 is compositional (紫 purple + 印 mark/seal/stamp) and circulates mainly as a brand or product name and as an ad-hoc descriptive compound for a purple blotch, brand-mark, or quarantine seal.