Macronutrients are essential elements that plants and organisms in aquariums need in relatively large quantities for growth and health. In the context of an aquarium, especially a planted tank, the primary macronutrients are:

1. Nitrogen (N): Essential for plant growth, nitrogen is a major component of chlorophyll, amino acids, and proteins. It is commonly added to aquariums in the form of nitrate (NO3-) or ammonium (NH4+).

2. Phosphorus (P): Important for energy transfer and storage, phosphorus is a part of ATP (adenosine triphosphate) and DNA. It is typically added in the form of phosphate (PO4^3-).

3. Potassium (K): Vital for the functioning of enzymes and the synthesis of proteins and carbohydrates, potassium helps in the regulation of osmotic balance and the activation of various plant enzymes.

These nutrients are often supplemented in aquariums through fertilizers to ensure that plants have adequate resources for optimal growth. It is important to monitor and maintain balanced levels of these macronutrients to avoid deficiencies or excesses, which can lead to poor plant health and algae growth.