I see definite signs of magnesium deficiency. If your tank is understocked, it's also likely that there's a macronutrient deficiency (nitrogen, phosphorus). From what I read online the fertilizer you use doesn't have any macros in it so it's possible there's some kind of imbalance there. It might be a good idea to stop dosing ferts altogether for a period of time and see how the plants respond. You'll have a clearer picture of what's going on and if the plants show any signs of unhappiness it will be easier to know which nutrients to give them and start the fertilizing regime from scratch. But I'm no expert, so take my advice with a pinch of salt & maybe someone will come along with more experience than me.
Water quality tests won't really tell you much in the case of aquatic plants, they're more important for the animals. What plants need is a good balance (and amount) of macro- and micronutrients in the water. Things like nitrates, nitrites and ammonia are only harmful to animals, but for plants they're a source of food.