Emersed to submerged growth is definitely a good thought.
I would also heavy consider a nutrient issue. Plants have the ability to cannibalize old nutrients to make new growth, but only some nutrients, namely all your macronutrients (nitrogen, phosphorus, and potassium) plus some micronutrients. This process can be very destructive to older growth, causing a variety off holes and/or discolorations in old leaves but leaving newer growth looking 'ok'.
Are you dosing any fertilizers with macronutrients (NPK)? You look like you have a fairly substantial stock, so this may or may not be the cause, but you almost certainly need some form of additional fertilization given those plants and that amount of lighting.
Algae creates 75% of the world's oxygen, so really we're all just doing our parts...