Well, true hyposalinity is 1.009. For this reason, I would still quarantine any and all fish. That is how low you must go for ich to stop breeding.
Also, DO NOT put any copper into your water. That is flat-out wrong. There IS some copper in natural seawater, but a very tiny, miniscule amount. If copper is being added to the water, there is already too much copper. Sea salts actually have a slight trace of copper in it.