The term that would best define the fish added to the pond is INTRODUCED species. These species may be harmful to native species.
Introduced species are exotic species which has arrived there by human activity, which may be harmful to native species.
Introduced species may even be harmful to the entire ecosystem because they can break the delicate equilibrium of the ecosystem.
Introduced species may compete with native species for limited resources (e.g., water resources, foods, habitat, etc).
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A scientific law
A scientific law is the result of consistent and repeated observation. Laws do not posit an explanation of experimental phenomena, they merely describe the phenomenon.
A few selected, and well known laws.......
Law of conservation of mass and energy
Newton's universal law of gravitation
Ohm's law relating current, voltage, and potential difference,