The less that goes into the tank, the more stable with the biology be. Plants require 17 nutrients, some they obtain from the fish and biological processes (carbon as CO2, nitrogen as ammonia/ammonium), some from fish food (certain minerals in fish waste), some from organics broken down by bacteria in the substrate, some in the water introduced as partial water changes. Rarely will all 17 nutrients occur from these sources, so we add what is probably missing via fertilizer.
Most missing nutrients are micro-nutrients or trace elements. These are all mineral, and include iron, copper, manganese, zinc, etc. Good liquid fertilizers contain these elements. Seachem's Flourish Comprehensive Supplement for the Planted Aquarium is one of the best, Kent's Freshwater Plant [not sure this is still available, haven't been able to find it for some time now] works (I used this in the 1990's), and I believe from the info on their website that Nutrafin's Plant-Gro is comparable, though I have not myself tried this. I currently use the Flourish.
Flourish Iron on its own I do not recommend. Nor do I recommend using Excel, which is carbon in liquid form. I'll explain why.
Plants need the 17 nutrients in quite specific proportions. Some of these nutrients, namely iron, copper, zinc, nickel and manganese are also known as heavy metals, and all are toxic to all organisms--fish, plants, humans--at certain levels. Therefore, we do not want to overload the aquarium with stuff like iron because in excess it will kill the plants and the fish. This is why good water conditioners detoxify heavy metals. Most water supplies contain trace elements of some heavy metals, depending upon the source. Liquid plant fertilizers contain these but at very low levels and in balance with each other to provide what the plants need without risking the fish and plants themselves. Excess of some nutrients can also cause plants to become deficient in others, not due to a lack of those but by virtue of the plant simply shutting down assimilation. As an example, an excess of magnesium causes plants to stop assimilating potassium. An excess of potassium cause an iron deficiency. And so forth. This is all why I advocate a balanced comprehensive micro-nutrient fertilizer.
The biology in an aquarium is a delicate natural balance between the fish, plants and bacteria, into which water parameters also are involved. It takes a few months for a new aquarium to reach its biological equilibrium which is a balanced state for the bio load. Adding stuff like Excel is increasing the carbon by a significant amount, and this will cause that balance to become unbalanced. Planted tank aquarists who add CO2 diffusion create a different balance, with increased (more intense) light to balance, and additional fertilizer. Takashi Amano uses CO2, high light, and fertilizes several products every day because as he himself admits it is essential to maintain the balance in his type of system. At the opposite extreme is my method of absolute minimal nutrients and light, sufficient to grow the plants but not beyond that level. Regardless of which method one uses, there must be a balance or it will not be successful. My process is to start with the least and work up to reach the balance; I have taken this approach because I believe that the less the aquarist interferes (by adding this, that and something else) the more nature will maintain the natural balance.
To conclude, I would use the Plant-Gro liquid as directed (presumably once a week). Monitor the plant growth over 1-2 weeks and be prepared to adjust if needed. I need to fertilize twice a week to keep my swords lush and thriving, because my water is so soft. Each aquarium is different, with its own unique balance. And we should all know by now what can happen when we interfere with nature's balance. As with the earth's environment, so too with the environment within the aquarium.