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Don't use beach sand directly - it's loaded with salt, debris, and organisms that'll mess up your water chemistry and potentially harm your fish. If you're set on beach sand, you'd need to rinse it thoroughly for a long time until the water runs clear, then bake it to sterilize it, which is honestly more work than just buying aquarium sand. Regular aquarium sand is cheap and already prepared for the job, so that's usually your best bet.
Beach sand contains shell fragments and crushed coral that can continuously raise pH and alkalinity even after rinsing, which is why it causes problems beyond just the initial salt and gunk. If you're really drawn to it, you could use a small amount mixed with regular aquarium sand as a decorative layer on top rather than as your substrate base - just keep it minimal and monitor your water parameters closely because it'll still leach minerals.
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