Originally Posted by trapperwolves
After doing some further reading I believe this could be a nitrogen or phosphate deficiency. Does that sound possible?
No. The problem is mainly the iron. You can see the brown patches on the leaves, within the yellow? Those are deposits of excess iron in the leaves. The potassium should not be a problem, most sources say that overdosing potassium will not cause problems.
The GH at 9, is this 9 dGH or 9 ppm? I assume the former, 9 dGH, so that should be fine for the "hard" minerals which are at insufficient levels in prepared fertilizers since the tap water supplements calcium, magnesium, etc.
To get this tank back to what it was, do once weekly water changes of half the tank, and the day following dose (only once) a 1/2 teaspoon of Flourish Comprehensive.
To the Hygrophila corymbosa. This plant needs good (= bright) light and high nutrients. I had no luck with it, it sort of "managed" for a few weeks, but then gradually declined with symptoms exactly as you describe. This was in a tank with two 4-foot tubes over it, but still not sufficient. Most stem plants are fast growing, and that means they need more light and more nutrients.
A single Life-Glo T8 tube is minimum over a 29g; mine is doing fine with this, but it is low to moderate light. Replace the tube after 12 months. This is one case where the less expensive Daylight tubes do not work well, they are not of sufficient intensity. The Life-Glo, and a couple others, are made for brighter intensity.
Back to the nitrogen and phosphate. The latter is usually more than sufficient from fish foods, even when not present in tap water [there might be some in yours]. Nitrogen occurs as ammonia and plants grab this and convert it to ammonium. You will rarely have nitrogen deficiencies in natural planted tanks with a fish population.
Final comment on iron. This often comes up, members thinking they need more iron. Your example is a prime one why this is never advisable, and why I always recommend against adding iron. Iron is only a micro-nutrient, like copper, zinc, manganese, nickel, and some others. Plus, they are heavy metals, highly toxic to all life forms at elevated levels. Flourish Comprehensive is a balanced nutrient supplement, meaning the various nutrients are in proportion to each other as required by plants. If the light is low or moderate, and CO2 is not being diffused, only in rare cases will additional supplementation of some nutrients beyond Flourish be necessary. And in most cases, iron will not be the one.