Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Fish food (and shrimp food) contain varying amounts of nutrients for the plants.
In general high protein foods (Piranha food) will have high nitrogen, high phosphate, moderate to low potassium and a reasonable amount of trace minerals, including some iron, but maybe not enough iron.
Plant based foods are lower in protein, so lower in nitrogen, but not too bad for the other nutrients.
When I had heavily stocked tanks I only had to dose carbon, potassium and iron, and I used Leaf Zone and Excel. I was basing the dosing on the nitrate level. As long as I had to do water changes to keep the nitrate low, I knew the plants did not need more nitrogen. Since plants use a low level of phosphate, I assumed that was OK, too.
You could try putting some slow release tablets under the substrate where the roots of the Amazon Sword can find it. They are very heavy feeders.