Thanks George. I think I've been put off for the time being though.
Storing for at least 3 weeks is fine but I wonder how 4-6 months would be?

George Patterson wrote:
Ed wrote:

Tullock's book
has a section somewhere in the beginning that talks about using
natural sea water.

Emmens "The Marine Aquarium in Theory and Practice" also has a section,
which has similar recommendations.

Emmens states that using the water immediately is "fairly safe, but risk
of disease is always present." If it cannot be used immediately, he
recommends storing it for at least 3 weeks in large containers in the
dark. When you're ready to use it, avoid disturbing any sediment which
may have precipitated out.

He also states that mildly polluted water frequently causes an increase
in ammonia and/or nitrite levels, and these levels can remain extremely
high for months, so test these levels before use.

George Patterson
Coffee is only a way of stealing time that should by rights belong to
your slightly older self.