I don't think the lights out treatment for 2-3 days will do anything for Dinoflagellates. You're going to have to starve them out. Very aggressive GFO will help a lot, along with filter socks, rinsing food well, reduced feeding, etc, etc.
Along with Phosphates, GFO also removes Silicates, so I would change it weekly for a month and see what happens. Have you checked your source water for Silicates?
BTW Dinoflagellates will have a whip tail under the microscope.
Rob
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