All plants need a diverse range of different nutrients in order to grow and flourish. The absence of only one vital element is sufficient to stunt plant growth. They may develop visible symptoms such as poor growth and pale, yellow leaves. Their natural resistance to infection declines and algae start to form on the leaves.
The situation with regard to nutrients in an aquarium is quite an unusual one: some nutrients, introduced in the form of fish food, are present in surplus quantities (phosphates, nitrates). Other nutrients are either consumed rapidly, or are not present at all. For example, tap water contains no iron – one of the most important trace elements. A good aquarium fertilizer will fill these nutrient gaps exactly as required.