Velvet is a single cell dinoflagellate named Amyloodiium Ocellatum. It is partially photosynthetic. I believe the term is mixiopic (but don't quote me on that) where it is photosynthetic and consumes outside nutrients. No need to turn of your lights. You are correct in that it needs a fish to complete its life cycle. It can live in the free swimming stage for a bit over 2 weeks without a fish host but that is it. Going fallow for 6 weeks will starve it out, lights on or off.