Potassium is classified as part of essential macronutrients for plants. Potassium allows the movement of water, nutrients and carbohydrates in the plant’s tissue. At the same time it also regulates the rate of photosynthesis.
Along with nitrogen, phosphorus is part of a very important group of macronutrients. It plays a role of regulating protein synthesizing which is important for cell division and tissue development. Plants with phosphorus deficiency often show symptoms of stunted growth of roots and leaves. In order for plants to effectively absorb phosphorus, a certain ratio between nitrogen and potassium have to be maintained.
Nitrogen is one of the main macronutrients in aquascaping. It plays an important role in maintaining the plant's health. Not only that nitrogen is a major component of chlorophyll (a compound that is used to absorb light during photosynthesis), but it is also a major component of amino acid which is the building block of protein.
Seachem Flourish Excel is design to supply carbon for aquatic plants to be use in photosynthesis. However it is not enough to replace the needs of CO2 injection in high-tech aquascaping. That being said, it will be a great help for low-tech aquascape that doesn't have CO2 injection.