Red tides are formed by dinofllagelata, eucaryotic flagellate organisms. Dinoflagellates are marine plankton which means they drift on the surface of the sea. Those are small, nonmobile organisms who sometimes bloom in concentrations of more than a million cells per ml. The bloom is often red colour so the phenomenon is called red tides. Under such circumstances, they can produce toxins, dinotoxins which are capable of killing fish, so that's why this occurrence is dangerous.