For healthy growth aquatic plants, Nitrogen, Phosphorous and Potassium are the three major nutrients for plants. Although Nitrogen and Phosphorous are produced from fish waste, remnants of fish food and dead leaves, but the Potassium is most insufficient and often lacking in the water. The 3 brothers (nutrients) play an important row in our tank for healthy growth of plants.

For the big brand fertilizer that buying from fish shop, mostly not included Nitrogen and Phosphorous and just added just a little bit of Potassium only. Unless those who planting Riccia and Glossostigma are often in shortage of Nitrogen and Phosphorous, and need to add more of it. Of course, the knowledge of putting a balance of necessary nutrients is important, and incorrect dosage will cause lots of algae problems.

From the JBL website link, know that high amount of PO4 and NO3 will cause algae growing. The excess PO4 and NO3 in the water are the algae nutrients.
http://www.jbl.de/uk/aqua/uk_algen.html#Generelle
And, as RuslanJamil requested about the link to those who taught in using Potassium as below:
http://www.aquabotanic.com/PMDDprimer.htm
All these are just a sharing references . Please check it out for more details.