Hi all, my friends and me are keeping Mediterranean cold water tanks so we take water directly along the coast, but here in Italy (and in southern France too) many reefkeepers use marine water without additional filtering. The only issue is the density, usually a bit higher in mediterranean water, so they have to add tap water or better RO water. Others, along the French Atlantic coast, uses ocean water as it is. So I believe you could use marine water wherever you can take it –OK, just as far as possible from rivers or lagoon outlets, from sewages or industrial areas and harbours!