Thanks for this detail; that helps--and I see a couple of things.
First, plants require a number of macro-nutrients and micro-nutrients in order to photosynthesize which is how they grow by converting sugars into energy. These nutrients must be in balance, and the light must be sufficient (in balance if you like) to permit photosynthesis.
The "dry" nutrients bother me; are they specifically intended for aquarium plants? Or are they garden or house plant fertilizers (some people try these)? The nutrients aquatic plants need are different in their proportions to each other from land plants. And there are so many different nutrients that making up all of them and in the correct balance is very difficult.
Seachem's Flourish Comprehensive Plant Supplement has this required balance; you can see the list on their website. I would strongly recommend you use only Flourish Comprehensive. Some nutrients supplied in excess can be stored by the plant, but this depends upon the plant species and the specific nutrient; studies have shown that an excess of certain nutrients can sometimes negatively affect the plant's ability to absorb other nutrients, leading to poor growth and plant demise.
It is also very difficult to diagnose particular plant problems. Aquarists sometimes see yellowing leaves and immediately assume iron deficiency. While this can be true, there are a number of other deficiencies that can cause yellowing leaves. And more iron may cause other problems. Using a comprehensive fertilizer solves these problems because everything the plants require is there and in balance.
Flourish Excel is a liquid carbon supplement. I believe AuntKymmie uses this along with Flourish Comprehensive and likes the results; I don't bother with Excel because I stock my aquaria heavily (lots of fish
) and they (plus the other biological processes ongoing in any aquarium) provide CO2 sufficient for the plants given the light and nutrients I supply. But using Excel is fine if you think your plants need it with your light.
On the question of the nitrite increase being the result of removing the filter floss from the canister filter--no, in my opinion. In a thickly planted aquarium, the plants remove most of the ammonia and nitrite before it ever gets to the bacteria, especially the bacteria in the filter which is far less than the bacteria colonizing all the surfaces including every plant leaf and every grain of substrate gravel or sand. Have a read of my post from Monday in which I went into this in detail, especially the last couple of paragraphs that deal with plant filtration: http://www.fishforum.com/freshwater-...ishless-27878/
Nitrates should be present in an aquarium, even a thickly planted one. Nitrates are not toxic to fish unless in very high numbers. Most authorities suggest that 40ppm is the upper limit for most fish (some can tolerate much more), but recommend 20ppm or less. In my aquaria nitrates consistently run between 5 and 10 ppm, and closer to 5 according to the API colour test chart. Partial water changes are the best way to remove/dilute nitrates; in a planted aquarium that is biologically established the nitrates will never be above 10ppm unless there is an adverse biological issue like neglecting regular pwc, overfeeding, or dying fish and plants.