Excess nitrates would not cause the diatoms but it can cause the cupping of the leaves. Some of the macrandra older leaves are deformed and shriveled which could be caused by nitrate tox inducing a Ca deficiency, but other nutrients in excess can cause the same symptoms, e.g. excess potassium.
In general, all nutrients must be in relative proportion to one another. While there are some that say nutrient ratios don't matter, this has no scientific support. E.g. some say excess phosphate doesn't cause harm, however, excess phosphate can induce micronutrient deficiencies and P tox in extreme excess. But similarly, excess micros can cause toxicity which can be partially relieved by adding phosphate back into proportion. This is well documented in the plant sciences literature, from farmers' experiences, as well as planted tank hobbyists.