The brown spots on the sword leaves are due to the accumulation of excess iron. In this instance, probably not because you are overdosing iron [the amount in Flourish is in proportion and not excessive], but because the hard minerals, especially calcium, is insufficient. [The GH of the tap water will confirm this.] Calcium is essential for cell structure in the leaves of aquatic plants, and it competes with heavy metals for assimilation. When calcium is lacking in the water, heavy metals such as iron are taken up by the plants in its place, resulting in an excess that is beyond the pants' capacity to store or process. The brown spots spread as more iron is taken up, until the leaf and eventually the entire plant is killed.
I had this very same issue and went ino this thoroughly. The calcium and other hard minerals as they are called are not in sufficient quantity in any of the standard liquid or substrate fertilizers, because these products are made with the assumption that most of us will have sufficient hard minerals in our water. I will go into this further when I have the GH number. It is always possble that something else is involved, though here I doubt it.