Cold water intertidal pool aquarium

[Waldo]

I've been keeping fresh water tropicals (Malawi cichlids) in a 1.5 metre tank and have now started to experiment with cold water marine setup. With fresh water you don't bother with gravity meters and protein skimmers. Regular cleaning of filter media and a 10% water change every 2 or 3 months is all the maintenance needed. My first experiment was with a garden pond filled with seawater mix made up to instructions in the sea salt package. It flopped. Now I have filled a 0.5 metre tank with actual sea water and stocked with some mussels (removed with minimal trauma, eg. removing rocks with mussels attached to them) and green seaweed. I use heavy filtration and aeration to copy the natural environment: My setup uses an underwater garden pond pump which pumps the water into the bottom of a filter box on the side of the tank. It flows up through porcelain rings and activated charcoal and cascades back into the tank. After three weeks the mussels are dying. Is the underwater pump motor poisoning the water with metal?