I've heard of people dropping salinity as low as 1.012 (even lower) to treat for ich and other parasites, and even one reefer whose tank was accidentally hypo'd w/ rodi overnight. Not all your livestock will like this -- inverts and corals wouldn't like it. Going down to 1.019 *might* be ok, but I am not sure how it will affect corals.
That is not a solution, though. The best solution is to QT everything, but if you haven't qt'd all your livestock by now, there's no point because any diseases are already in the tank. But unless stressed, the disease won't present itself. And usually, fish can fight it off if your water is really good. A lot of fish are already carriers of ich and/or parasites, but since they are not stressed, you won't see it.
As bad as it is, some of us here do not qt, but we understand the risk of diseases and deal with it. But really, a fish can fight off the diseases on their own, especially if you boost their immune system w/ garlic drops and vitamins. The more they fight it, the more resilient they become to the disease.
Those who QT everything from the start have good results, of course, because the disease is not given a chance to enter the tank. They qt ALL the fish.
I doubt your ray died of disease, but rather from stress from high nitrates.
