Do you have enemies? Not sure how to deal with them? Have you thought about eels? It's classical!
The Roman noble Vedius Pollio kept a pool of large eels, & he threw in people who displeased him, then watched the eels tear them apart.
(read to the end for a demo video). 1/4
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Now...there's some disagreement about the kind of fish Pollio kept. The latin word is murenae, which can mean lamprey, eel, or moray.
Lamprey is the most frequent translation, 'cause they're scary looking. But they're not lethal. Eels or morays make more sense. 2/4
I agree that for people eating purposes morays make the most sense. But I am skeptical about the technological ability to keep saltwater fish alive long enough to meet unpleasant guests at any time.