[Ameivaboy]

I would not feed anything from salt water myself as I am unaware of how garters would rid themselves of the excess salt (like iguanas are known to sneeze stuff out)

as far as parasites and freezing, it's suggested to freeze for 30 days but that DOES NOT elimanate all types of parasites. Garters and pretty much any other species can live with small amounts of parasites with no ill effects the problem lying in unsanitary caging conditions which the animal keeps reintroducing the parasites into it's body and stress which weaken the immune system allowing said parasites to take over