If your tests all show quality then it is strange that you are suffering losses unless there is a basic health or environmental problem. I assume routine seawater changes are being done and that all the aquarium equipment is running as it should, including the protein skimmer.

If you believe there is ‘gilvin’ present then there isn’t a reason why activated carbon marine grade shouldn’t be used. Try running with it for a week. Remember that activated carbon does remove some trace elements so it would be a good idea to do a routine seawater change after the week. ‘Gilvin’ presence is not a disease but a seawter condition.
.-= John´s last blog ..Aggression On The Reef =-.

For example: HBH I would consider a low grade, Black Diamond a Medium Grade, and then I use a ROX carbon which would be considered high grade.

For some reason aquarists have this idea that when they are rinsing carbon they need to grind it together. This is not necessary and only creates more dust. Just rinse it well under cold water until the water becomes clear. Remember to rinse from all angles.