Tetra fish probably are felt to pose a high risk of becoming established in the local waterways. Additionally, they may harbor diseases which could be transmitted to native fish.
Water buffalo are used to plow fields in Asia, and farmers who have some unusual livestock may want to have them. Since that's a domesticated animal, and poses relatively little threat of establishing a wild breeding population, there's no reason to bar them. They're vaccinated the way cattle are.
So, the real concern here isn't a danger to people, it's a danger to local wildlife.
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