arsenic is a naturally occuring substance in sea water and thus will appear in all sea products, including sea vegetables and fish. arsenic will naturally appear in trace amounts in much spring water. in fact, there are acceptable levels of arsenic in drinking water across the US. most soil has some arsenic in it.
this is fear tactics and much ado about nothing.
the japanese eat kelp and seaweed for breakfast lunch and dinner. are they all keeling over from arsenic poisoning? no, in fact they are the longest living people on earth.