I was wondering if anyone had any good input or links on the role sodium plays in our tanks.
An imbalance between sodium and potassium is one of the quicker ways to brown off plants. Get some of the salt they make for folks on a low-sodium diet. One brand name here is "No Salt" but your grocery store probably has a salt substitute containing mostly potassium chloride.
Use a bit of it in the tank with the browning plants and see if it stops further damage. It only takes a modest amount, so don't kill things by overdoing it. I would guess at about 1 tsp. per 10G is plenty -- less in really low tds water.