Originally Posted by GwenInNM
I have the last one you mentioned - a bottle each of nitrogen, phosp and Potassium.
My tanks are not close to be overstocked and I do WC 2 times a week, so I'm always trying to keep nitrates at or below 10, but I have seen them creep up to 20, that is why I'm wondering if I'm overfeeding or what. Still trying to find the balance and ideally I'd like to get to just once a week water changes :)
I think this is the problem with the appearance of the Hygrophila corymbosa "angustifolia." It is not receiving the full balance of essential nutrients, but is receiving an excess of a couple which is actually causing more harm than good. I'll explain.
There are 17 essential nutrients required by aquatic plants, and these are in a sort of proportion to each other. For example, carbon is the nutrient required most by which I mean the greatest quantity. As soon as one nutrient is no longer available to the plant, having more of any of the others will not achieve optimum growth because the missing nutrient will hold the plant back, so to speak; we call this the limiting factor.
Aside from this, the excess nutrients can sometimes cause further difficulties for the plants. For example, some are known to cause plants to shut down assimilation of other nutrients. Some authors say this is because the nutrients are in competition and one wins out. This is why it is important not to "overdose" any nutrients. And, some of them are highly toxic to fish and plants in excess. Also, plants generally take up what is present whether it is actually needed or not. For example, if copper is available, plants will take it up; but they need very little as a nutrient, and the rest causes metal toxicity which manifests itself in brown spots and the breakup of plant tissue. However, other factors also cause brown spots, so don't jump to conclusions.
If you had both of the Seachem packs, you would have green plants. This plant is a fast grower, like most stem plants, and that means it requires more nutrients than plants such as Anubias, Java Fern, crypts, and so forth. But in your situation it is missing some very essential nutrients, namely carbon and trace elements. Now some of these will naturally occur in the aquarium, some may be present in the tap water, and some may enter via fish foods. But not all of them can, nor can we assume the level of those present will be sufficient either to be used by the plant or in balance with everything else.
Carbon occurs as CO2 from fish and bacteria; more comes from the substrate bacteria than fish. This is usually the nutrient that is in least supply, which is why adding CO2 generally improves the plant growth by allowing the plant to grow faster. But in most aquaria, there is more CO2 than aquarists often realize, so it is certainly possible to have good plant growth, although slower, without adding CO2. And the problem with adding CO2 is that the light has to be increased as well, along with all the other nutrients in proportion. Again that balance.
Trace elements are missing from your present fertilization and these are absolutely essential. One of these, iron, though required in relatively small amounts, is essential for chlorophyll synthesis, and of course chlorophyll makes leaves green. Manganese is also essential to activate enzymes that are used in chlorophyll [that green again] and photosynthesis. Zinc is another used in chlorophyll production. Molybdenum is needed for the enzymes that convert nitrate into ammonium so the plant can then assimilate the ammonium which is its prime source of nitrogen. So adding nitrogen as you have been is not going to help because the trace mineral molybdenum is not present to allow the plant to use the nitrogen which it must first convert to ammonium. This is also partly the cause of the nitrates rising. The plants cannot use them.
Your plan to use only Flourish Comprehensive Supplement will undoubtedly get the Hygrophila green again. At this stage, because of all the macro-nutrients present, I would only use the Flourish Comprehensive for a few weeks. Later, depending upon the response of the plants, perhaps adding the Flourish Trace Elements in addition may be necessary. I don't use the latter, just Flourish, but then I have almost entirely relatively slower-growing plants than the stem plants. I do have Pennywort which is thriving like a weed, but it is a stem plant that manages with less of everything including light. And as I mentioned previously, my Hygrophila corymbosa is growing but very slowly, probably due to the minimal light and nutrients.
Last comment on nitrates. In a planted aquzarium, nitrates should be steady and at a low level. My tanks are all between 0-5 nitrates and have been for years. Kymmie's i believe she said run at zero nitrate, and that is very common in natural planted tanks. When you see nitrates rising above 10ppm in a natural (low-tech) system, it means something is out of balance.
Hope this has helped.