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Species will determine the spacing. As mentioned, keep LPS and SPS several inches apart. Most LPS also have long sweeper tentacles that come out at night, so care should be given when placing them next to any other sensile invert.
Not just LPS and SPS but some soft corals also. Leathers come to mind as they will engage in chemical warfare if close to each other. As stated earlier by Ziggy and Brian LPS need plenty of space as they have sweeper tentacles that will sting other corals.