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“Not just anywhere” is an expression used to describe a very specific set of requirements related to a location. For example, grapes used to produce wine cannot be grown just anywhere. They need the right soil and climate conditions. In this case “just anywhere” is used to emphasize that there are specific local requirements.
It’s the same with fish. Salmon have very specific needs (temperature, current, salinity). You can’t raise salmon just anywhere: you need to have the correct conditions for them to grow.
Do you know people who are car lovers? If something goes wrong with their cars, they don’t take it just anywhere to be repaired. They take it only to authorized dealers or high-end service stations (no quick-lube centers for them!).
Have you ever run into a movie star at your barbershop or hair salon? (We haven’t!) Movie stars don’t get their hair cut just anywhere. They go to specialized salons to maintain their image.